If you are not satisfied with your amp or dac-to-amp combo, then you might want to upgrade the amp, but it depends on what you are trying to accomplish or achieve sonic-wise.
If your amp is ok, and you are looking to add a preamp, then add a preamp.
The BHK250 is also on a closeout sale right now from PS Audio, so now is a good time to get a brand new BHK250.
My two cents from owning the DS, the BHK preamp and the BHK 250 and still owning the transformer upgraded DS. If you like the sound of your Quicksilver amps with the GR Research speakers they have more than enough power and the tube magic that work with Danny’s higher efficiency models. I’ve tried the DS over the last seven years directly connected to multiple amps including the 250 and found the sound to be flat and uninvolving. So since you like the sound of the amps and the DAC I would look at a preamp first. In your situation and with your gear the BHK preamp would not be my choice after owning it for four years and being the first piece of the three to be sold. From years of being a member of this forum and others the sample to sample difference in the preamp is pretty significant including parts and construction changes. From my experience and that of friends that owned it and sold it the BHK preamp doesn’t play nice with amps that are also not fully balanced unless you are ok with loud clicking noises every time you adjust the volume. In addition it is the most tube finicky tubed preamp I have ever encountered and created the most self noise. If I was looking to buy a preamp to go with your amp and speakers I would look at for tube preamps the Supratek Chardonnay and Cabernet, Aric Audio, Linear Tube Audio and Backert Labs. For SS I would look at the Holo Serene KTE or the Wyred 4 Sound STP-SE Stage 2. I suspect that shortly the “usual suspects” will jump in to contradict me but my opinion is based on actual experience and not being a brand loyalty enthusiast. Also if you don’t have a problem based on your location with the heat generated by the BHK 250 or the weight and bulk the 250 is a great amp. The other factors and not the sound are why I reluctantly let mine go.
I have tried many a DAC straight to the amplifiers. My very strong preference is use a Preamp between them. I have never been disappointed from putting a Preamp between my DAC and my amp.
I have a nearly new Backert Labs Rhumba 1.3 Extreme and it is a killer piece. Dead quiet, rock solid and simple to use.
With my recent downsizing effort it will be showing up at TMR soon.
My situation is, not only has the preamp greatly improves the sonic presentation in my system, it also gets the level of volume position where it is ideal. But you have to get a high enough quality preamp at lease like the BHK pre or the sound will be compromised. I would get the power amp first and if it mates well without a preamp, and you don’t need an analog imput, by all means forget about the preamp.
I used the DirectStream Mk I direct to the BHK250 via XLR for years. At the urging of this forum, I tried the BHK Pre and no doubt, it’s better. How much better? Only your ears and your budget can decide. If the BHK250 is your ultimate goal, I would grab one at the close out price. One more thing… the BHK250 seems overkill with those efficient speakers.