Crazy4blues - You owe me a burrito! The first two years of owning the 2.5s I had a Mesa Baron 150wt tube amp and have also used a 150wt SS and 25wt various tube and SS amps. The ProAcs sound best with the Air Tight ATM300 but, again, they are a very easy load, I listen in the relative nearfield, and my room is small. I know exactly what you mean, though, as I've gotten subtantially better bass from them and that's why about two years ago I bought a REL subwoofer. Tube and especially SETs sound far more powerful than the simple wattage rating. The SS Muse mono 150s never did sound powerful since the music just got louder, it didn't get more, um, "musical", if you know what I mean. I think that's why we all have tube amps.
And I know what Phil means since I've also owned 572 SETs which put out 15-20 wts and they drive the ProAcs wonderfully with deep and well defined bass. They actually did more realistic bass than the Mesa which simply pounded the bass region harder and louded.
I use a Placette remote volume control instead of pre-amp as I've also tried a bunch of those and even owned an $8,500.00 tube job. Nothing sounds more direct and transparent than what's being used now. (Hey, where's that guy who used to tell us we have to spend $15,000.00 to get a proper pre-amp?) A pre-amp will add another gain stage which may likely make any noise louder, if it doesn't add it's own.
I'd like to spend about $2,000.00 for a used tube amp and have been checking out various ALSs and ANs but, unfortunately, looks now matter to me so it's taking more time than it should. I'm havin' fun, though!