I have a BHK Signature Preamplifier and BHK Signature 300 Mono Amplifiers on the way. There are only one pair of XLR outputs on the preamp. Does this mean I will have to use the one of the RCA outputs to hook up to my subwoofer?
Ideally, drive your subs high-level from the 300s. Jensen makes a nice 2 channel transformer if your sub doesn’t have high-level inputs.
Otherwise, it won’t make much difference fed by balanced or single-ended, unless you have very long runs and need common mode rejection of balanced.
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You might want to go hi line out off the BHK300…and better put by Brett. Especially about single ended and common mode and RCA cable length.
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Ideally, drive your subs high-level from the 300s. Jensen makes a nice 2 channel transformer if your sub doesn’t have high-level inputs.
Otherwise, it won’t make much difference fed by balanced or single-ended, unless you have very long runs and need common mode rejection of balanced
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Forgive my ignorance but how do I accomplish using the high level from the 300’s.
BTW it will only be about 6 feet from the amps to the subs.
The subwoofer is the JL Audio f113.
Thanks
Fathom® f113-SATIN
Powered Subwoofer with 13.5-inch Subwoofer Driver, 2500 watts
The Fathom f113 is a heavyweight subwoofer capable of filling the most demanding rooms with powerful low-frequency output and reference-class sound quality. The combination of an ultra-long throw W7 13.5-inch driver and a very powerful switching amplifier give it the kind of output normally associated with far larger subwoofer systems.
Because it is built with the same driver and amplifier technology as our flagship Gotham®, the f113 delivers a tremendous level of sound quality. Low distortion, excellent dynamic capabilities and shockingly deep bass extension will allow you to experience all the excitement of the most demanding cinematic material, while also being able to reproduce all the nuance and texture of your most delicate musical material.
A complete set of signal processing features is easily accessible on the front surface of every Fathom. These include a highly flexible low-pass filter, variable phase, switchable polarity, e.l.f. trim and our powerful Automatic Room Optimization (A.R.O.) system. A calibrated microphone is included for the A.R.O. system. Input connections are made via unbalanced RCA connections or balanced Neutrik® combo XLR/TRS jacks. Also included is an XLR output to connect a second Fathom® as a slave unit.
Looks like no hi line input. Nice thing about hi line is that your main speakers see the exact same signal as the subs see.
high-level is another way to say speaker-level. One simply runs another cable from each 300 to the sub as if it was another speaker.
You will need one of these as Cardri posted your sub doesn’t have a speaker-level input. Nor do mine twin subs, this works great.
Jensen Iso-max Model SP-2SX
https://www.cs1.net/products/jensen_transformers/SP-2SX.htm
The integration of subs with your mains will be so much more seamless with the transformer and it’s a very small price to pay compared to your find BHK pieces.
What main speakers do you have?
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Thanks so much Brett and Cardri!
I have Von Schweikert VR-5 Anniversary MK II speakers
What type of connector is on the input of the Jensen Transformers SP-2SX?
I will be bi-wiring my VSA speakers so I will be using both outputs on the 300’s. I’ll assume I can just add another cable to one of the outputs. So on the amp end it will be spades. What about the Jensen Transformers SP-2SX input end? As stated above What type of connector is on the input of the Jensen Transformers SP-2SX?
Have a look at this one, RCA out with a terminal strip in. This is what I have not the XLR model. The cable used from amp to the transformer terminal truly doesn’t matter, something easy to work with like zip cord works great. It’s not interacting like it’s connected to a speaker…it’s simply a conductor.
Thanks Brett…I think I’m starting to get it now. Looks like I can connect to the output speaker terminal of the 300 (spade) to the Jensen Iso-max Model SP-2SX which uses a Neutrik SpeakON® input connection.
So basically a Spade output to Neutrik SpeakON input connection.
I think Your next question should be: “Brett, should I hook up xfmr out to sub stereo in or mono in and also do I connect the XLR between the subs (slave…) or just cable L & R from xfmr out to each sub mono in…?”
Very nice main speakers, can’t go wrong with thay set up… Well done!
Yes, then XLR or RCA out to sub input. I don’t think it matters much if connected in stereo or not, but I’d do stereo, just because. I think it’s more important to place the sub(s) in the room which it sounds the best and this is likely not near the main speakers, as highs and mids behave very differently than lows.
And especially if you mains go low, I want the subs in a complementary location to excite more room modes and thus smooth the bass overall in the space.
Thanks to both of you. It’s clear to me now. 
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After many places including the corner, I found on the inside rear of the mains so they are near the launch point of the main woofers. That area just sounded the best. In the end you want to match the subs to the mains in a way that when you close your eyes and listen you can’t tell there are two sets of speakers. Take your time with speaker placement, they are your system, they define your system. At some point you may want to sweep your room (acoustic measurement) to get an idea of the acoustic treatments you might want to employ. That would be another thread for another day. There are many very knowledgeable audiophiles on this forum and love to help other audiophiles… 
In the end it is about loving to listen to music…
Just for completeness, I assume you are bi-wiring your main speakers? Vendor recommends and ask @Gary_M about if it matters… He found 30% increase in performance.
Yes, I am bi-wiring with 2 direct runs to each speaker of Wireworld Silver Eclipse speaker cables. It absolutely makes a big difference.
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I just ordered the Jensen Transformers SP-2SX Stereo Speaker to Line Audio Converter. Thanks for all the replies.
So the SP-2SX doesn’t come with the with Neutrik SpeakON input cable. What are you guys using?
According to Jensen, the SX does have Speakon input connectors. The NR has a terminal block. I have the NR version and just use zip cord to go from amp to transformer.
Thanks Brett!!