One of my favourite sounds are the times I used my little Eico HF81 14/w side classic tube amp on my big Thiel 3.7s!
It was utterly glorious - rich, open, detailed, sparkly transients - they sounded even bigger than they ever had before.
Really the only negative...if it was a negative...was that the bass became a bit "bigger." (I'm presuming from less damping). I say "bigger" not "looser" because the bass didn't really become loose or flubby so much as just...bigger and richer...a bigger bass booty ;-)
Ultimately my CJ Premier 12s and pre-amp make for a bit more even sound top to bottom and allow for easier control of the system (e.g. my pre-amp has remote control which I need as my amps are in a separate room from my speakers). But if that weren't the case, I would have used that little eico on the big Thiels more often.
I still have to get around to trying it on the 2.7s! (But..well...that project is now delayed...)