Tube/SS synergy?

Well your sense of compression could also be caused from this.
It seems that compression is all the rage in the recording industry nowadays.

Cheers,
John

The input impedence of the amp should be at least 10 times greater than the output impedence of the preamp .

Joeb, is your LS-15 with REL caps or InfiniCaps?

I had LS-15 with REL caps and it severely lacked bottom end punch. I replaced the caps with TRT Dynamicaps and it changed the sound entirely. It had more punch, smoother and overall more natural. The difference was substantial.

I now use ARC LS-25MkI(have to say that LS-15 with Dynamicaps come extremely close to LS-25 in sound) with X250.5 into N803 and it is very good. I use ARC CD3MkII cd player. All balanced cables. By the way, in balanced configuration you will get higher output from your LS-15 into Pass amp and it may improve bass output plus you will lower the overall noise floor. ARC is meant to run balanced. Also, before giving up entirely on the LS-15, if yours is with InfiniCaps(better version) try some NOS tubes if you are using stock tubes. Sovteks or EH tubes do not have very good bass drive.

Good luck.

If you really want to stick with a tube pre, you may still be able to do so, though the options are limited. The better Sonic Frontier pre's are DC coupled low output impedance and I beleive the Atma-Sphere pre's might work too. There may be others. I have no personal experience with the Atma-Sphere, as interesting as they are, they're beyond my budget. I should warn you, that to some the Sonic Frontiers are not as tubey as most other tube pre's, still one of my favorites. Bob, offers a really good recommendation. The Pass pre's are IMHO excellent.