Pure preamps are simple in design but the markups are huge with pretty knobs, switchesand case.
I would focus on speaker replacement and get higher powered amplification depending
on the speaker sensitivity, etc. Perhaps even biamp as a minimum. I think the
speakers are your weakest link, then amplifier, the rest is fine and does the job well.
Since you have access to proaudio amps, you might want to do a test. See if you can
score a PLX for testing and bridge the amp and drive one B&W cabinet to compare how
the speaker is being driven vs. the Rotel amp. You want to rule out the amplifier as a
variable meaning you want to ensure the speakers are not be starved for power.
If for some reason, the speakers sound better with that test, then keep the speakers
and do the higher power amp upgrade. If you think the speakers are not offereing anything
more to the party, then the speakers need replacement.
You can always enter DIY speakers but that is a dfferent can of worms to deal with.
It will take time and money and experimentation to find your sound.