best bang for buck on used ht speakers? maybe b&w?

I am one of those people who likes to upgrade once and not look back for many years to come. If you are serious about having a SOTA, long-term system and wish to make big jumps in quality, the B&W 800, 801 and 802 are serious speakers that could last you a long, long time. With the 800 and 801, you will want to have big-time power to drive them...not just watts...think current...high current. If you are going to be in this hobby for a while, and if your amp can drive the bigger B&Ws "okay" for now...i would do that...and then next i would look to upgrade the amp. Trust me - a really powerful, high quality amp will have transformational affect on your B&W 800, 801, 802. Those speakers can absorb power...a lot of it...and continue to improve with better and better power. the CDMs could move to the rear, and pehaps you could keep the 602s in a second system or trade them in to your dealer as part of your next purchase. What you will quickly find is that the sub goes thereafter...and then you are into the SOTA arena.
my two cents. enjoy!!!

Definitive Tecnology

I just sold B & W Matrix 803's. I had the same Pioneer receiver and then used it a the pre - pro and upgraded to an Emotiva XPA-5. The B & W's performed very well for me for about 20 years. I sold them for $800 and I feel the buyer purchased a great speaker for that price.

at what point does the cost of a newer high end speaker become only something that a special treated high end room would be benefited from? or for that matter something that only a trained high end person would be able to benefit from?

Baran:

I nearly have the same as you and upgraded over time. I have the Pioneer Elite 94TXH as it had 140 wpc for my 602s..I bought a pair of CDM 7NTs and replaced those fronts with those CDMs and it made the world of difference especially in the bass dept. I was also lucky enought to score a B&K reference 200.7 (seven channels at 200 wpc) to use pre outs on the Elite...MAJOR DIFFERENCE...the B&Ws loved the added juice. If you can find a multi channel power amp and use the pre outs off the Elite your speakers will love you as well as your ears.