recommend full featured preamps?

Have you considered 'tuning' your room?
I know what it is like to be space/layout constrained, but just maybe, a few small adjustments would help.
Something for first reflection....a dense wool tapestry like I use or some carpet directly in front of the speakers.
Extra points scored for good imagination here.

Just may help!

My speakers are Infinity Prelude PF-R, which are highly efficient - but I think their impedence curve does dip down to almost 3 ohms at one point. The Baron is not like other tube amps, its output is more dependent on the speakers' imp. curve., but seems to work ok (it is very adjustable). The Preludes do have switches for adjusting the bass output, but they just make it boomy - the wrong curve for my needs, I guess. I do have a rug in front of the speakers, but more work on early reflections is probably a good idea. TV monitor is between them, unfortunately, but speakers are forward enough to avoid 1st reflections from tv's edges. Thanks to everyone for all the thought and suggestions.

"I think their impedence curve does dip down to almost 3 ohms at one point."

That's probably a red flag with most any tube amp.

What is the amps rated output in 8, 4 and 2 ohms? If it does not double each time or come close, that could well account for the poor bass response.

Roger Modgeski of Music Reference/RAM Labs is working on a tube preamp in the $1,000 range that uses tone controls. He was looking to design controls that overcame the problems of tone controls and the sonic damage they do. I think they may actually be available now. I don't know if he succeeded or not, but I do know he is one heck of a good audio engineer. I'm sure he would be happy to talk to you about. I think it is part of his SET series of components.