Tubes vs Solid State

If those 802 d's and 800 diamonds are the B&W's I think they are, then you're probably going to regret going from ss to tubes.

If you can live with the practicality issues of that ARC 600 (ie size, mass, heat generation, tube life, etc) and cost (isn't it about 10x the JC1?), then go for it. For that kind of cash, I would have your dealer install it in your home for a trail listen. Then you can see if you can live with it.

Generally, I have found SS amps to be better for me (and I also own a pair of JC1s driving B&W 803ds) than EQUIVALENT priced tube stuff, this comparison is a whole new ballgame. Generally that is because driving my 803ds with an equvalently priced tube equipment means a lot lower power in the tube amp which results in lower dynamic range and transient punch. Where power is important (particularly with B&Ws and movie sound tracks), solid state is the cost effective winner. The ARC 600 takes the power issue out of the discussion, even if its not an apples to apples comparison.

Going to a tube preamp is nothing big deal.
But going to a BIG tube amplifier IS a BIG DEAL.
I would love to have a pair of ARC REF600s to run my speakers..
But the power tubes are a big drawback.
If you are buying used, then you may have an issue with the tubes soon enough. Sending them back to ARC for retubing is a must unless you know and trust a local tube guru.
Personally if I have the money to buy the ARC REF600 i WOULD certainly do so for the great experience it could be.

So I say if you can afford the expense of retubing, go for the ARC anyday. It may be a really great experience.
(Or a moneypit)

Matching amps and speakers is important!

Take a look at this link:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

The B&W 802 is designed to work on an amplifier that can double its power when the load is cut in half. Specifically, it has an 8 ohm midrange and tweeter, but the woofers are in parallel and so are 4 ohms. Not only that, but the woofers are 3 db less efficient than the mid and tweeter. To correct this, its expected that the amplifier will double its power into the lower impedance.

No tube amp can do this and so on this speaker, tube amps will have less punch in the bass. Now if you had a speaker that did not expect that of the amplifier, then you could get more punch out of a tube amp than a transistor amp...

Its all in the match. Of course its my opinion that tubes offer more music much easier than transistors do (I know of one transistor amp that is really musical but it retails for over $100,000...).

Another issue with 4 ohms is that the speaker cable is more critical and also that any amplifier driving 4 ohms (tube or transistor) will not sound as good as it does driving higher impedances. So if you are seeking musicality, you may want to consider replacing the speaker as well.

If you need to stick to the B&W's, stick with SS. In addition to all the reasons stated, tubes will be unstable with the loads the B&W need. I had a similar problem with my B&W's but I then sold them off to get speakers that were tube friendly.