I can't tell you how the 450 compares to your Crown, but I have the 240/3 for my HT system and the 400R for my two-channel
system. Both are excellent amps. I bought the 240 first just to see if it was as good as Frank claimed. Honestly, I wasn't expecting a big jump in SQ, but boy was I wrong. My left, right and center speakers really hop now.
I absolutely agree with what Larry said above. If the 400R (or 600R) is in your budget, definitely go for it. The FET Valve amps are clearly superior. That said, if you can get a 2nd hand 450 for $1200 (and the 400R is a little out of reach), run....don't walk to snap it up.
Before I bought either AVA amp, I had a SimAudio i3.3 integrated amp for 2-channel. My 240/3 amp paired with a NuForce AVP-18 pre/pro blew past the i3.3. I had not expected that. I had placed the SimAudio on a pedestal. Imagine my surprise when an amp that costs less than 1/2 of the cost beat its pants off and sent it running. Granted, we're comparing an integrated with separates, but this gives you an idea.
I was so impressed that I knew I'd move up to the 400R much sooner than I had planned. I'm so happy that I did. Some day I will upgrade my T8 preamp to the FET Valve Hybrid preamp. I nearly did so a month ago, but financial reality got in my way. (Sorry, Frank.)
Michael