@klhrw1

The EVO 400 replaces the HP. It has 8 power tubes. There should be no issues connecting the previous generation preamp to the EVO other than the previous generation preamp does not have balanced and the EVO 400 amp does.
@ghasley - Thanks, indeed there're 8 tubes.  So much for my college algebra ;-) 
I recently bought an  EVO 400  PL  preamp  which is connected to an Audio research VS-115 amp with  Silnote Morpheus Reference II silver XLR's . An amazing combo.  Excellent bass, midrange and top end extension. Very quiet preamp. 
For what its worth, I have the EVO400 Integrated; now running KT150s, and Mullard tubes on the two front; it's glorious.

I spent a few weeks with the stock EL34s; and their midrange and holographic presentation is undeniable, but I prefer the extra "oomph" of the big KT150s; I didn't realize I was adding a space heater to my room but se la vie.

THIS is the sound I've been after for 40 years.. so beyond thrilled with the amp; also nice Im in SoCal so a trip to Upscale is only 45 min.. comparing to other amps its a no-brainer IMHO.. best bang for the buck in tube land. 
curio -

I could not be happier with the Evo 400 preamp. The 400 power amps as well. The choice of balanced / unbalanced inputs and outputs is great to have. The sound I’m getting is ultra-realistic and full bodied.

I am using expensive power cables and interconnects, and I have applied my favourite tweeks to the PL chassis and connections (well documented by me in several of my recent posts). I’ve really pushed the limit as far as I can. The upshot is, the Prima Lunas are magnificent straight out of the box, and can easily be made significantly better if you are so inclined. I like that aspect.

The build quality and the quality of components inside the amps are second to none. I really can’t imagine anyone being disappointed with the PrimaLuna level of quality and construction. I’m so glad I decided to go with the Evos - they are up there with the best I have ever heard, at a price that makes you wonder how some other brands can even survive against that kind of competition. Some may prefer built in the USA (but not for quality reasons, I venture to add).