Welcome to this growing and cheerful gathering, Bill M, and congratulations on your order of the Double Impacts. Glad to hear that you are also considering the LM 508ia integrated, which I am enjoying so much that I have not been the least bit tempted to try a preamp with it. I think you'll find it is very good with the stock tubes, but shockingly good once you roll in the tubes to suit your listening preferences.
In my unit, I am using EML XLS 300B tubes, which are not exactly cheap, but are built to last and sound phenomenal in the 508. I replaced the 6SN7 tubes with Sylvania VT-231 military type tubes from the 1940s that I purchased from Brent Jessee, along with an RCA VT-229 to replace the 6SL7 stock tube. Each of these changes resulted in a very noticeable improvement in tone, texture, and layering.
I have yet to roll the 805 tubes, but will probably give the Psvane 805s a whirl later in the summer or early fall. I am so pleased with the sound at the moment, I am in no particular hurry to mess with it.
As for an earlier question pertaining to digital and/or analog, I greatly enjoy both in my system. I would say that I am fairly close to Kenny's breakdown of 70 percent digital to 30 percent analog.
Please keep us posted on your progress in building your system. If you need help finding a 508ia, let me know and I will hook you up with a dealer I found here in North Carolina. He's an incredibly nice fellow.
Chris