Thanks for the shout out, Charles. Yes, I think the Line Magnetic 508ia is an incredible amplifier, so much so that it forced me to reconsider my previous bias (no pun intended) for separates. I do indeed believe that the pre section is pretty great, though I may at some point do some experimentation with a good preamp to see if further improvements can be had by adding a quality pre in the chain. That has always been my experience in the past, but as I said, the LM 508 has resulted in a paradigm shift of what an integrated can do.
I do think it is necessary to upgrade the stock tubes to get the absolute most out of the 508, especially the 300B and 6SN7 tubes, though it also sounds good with the stock tubes. Adding some EML 300Bs and a nice pair of Sylvanias or the 6SN7 tube of your choice will yield very noticeable improvements.
To my ears, it is a stunning partner for the Double Impacts. I've had systems that were more than twice, even three times, as expensive, but I have never had better sound than I have now.