Are there any mods done to the BHK Signature preamp making it more enjoyable?
If so, please share your mods. There is so much on the DSD dac MK1 being done, like to ask about the BHK Pre.
The best upgrade MOD is probably a trade-in. 
PMG is big jump and not incremetal to the BHK ?
Depends greatly on the rest of one’s system. Trading one very good for a next level component can be “meh” or “wow” depending on the resolving abilities of the rest of the audio chain.
My experience from upgrading BHK DS MK2 APS to a PMG 512 has been well worth it, and the biggest difference is on the lower background noise; PMG 512 reveals so much more. But I believe BHK pre upgrade to PMG pre may provide even a bigger improvement since BHK pre has been around for a long time unlike DS MK2.
I did own a BHK pre, but I upgraded to a Grimm MU2 which includes an excellent analog volume control. Later on I upgraded PSA M1200 amp to an Gryphon D333 integrated amp that brought my system to a different level. These pre upgrades made me realized how noisy BHK pre sounded, and I am not sure any MOD (have not heard any) will possible improve on that. PMG pre will be the easiest recommendation to make on SQ upgrade.
I believe this about the BHK Pre as you say, but not having heard anything better in my system I am trying to continue to enjoy the BHK Pre. However, I have to admit that if I were to do any upgrades, it would be BHK Pre to PMG pre.
BHK pre is a great sounding pre and it was the flagship product before, but it has been around for over a dozen years. I did find tubes and fuses could improve its SQ to a certain degree. However, it was not able to reveal the best SQ in my system as I found out later.
I agree to upgrade BHK pre first will be the right decision and PMG pre may be the best way to go considering better trade-in value.
Over the years I don’t recall any mods being discussed to the BHK pre except tubes, fuses and power cables. They all helped but the PMG pre stock bests the BHK easily. Save your money and invest in the PMG pre when you are financially ready. And thank us later!
Future Tom thanks you all.
Just wait until future Tom gets some FR20’s, lol