What should I upgrade next?

I thank all of you for your input. Listening to all your suggestions is why I am have the next step problem trying to better my system. After all this I think I am heading toward doubling my budget $2000-3,000 used and buying speakers. Electrostatic most likely among the leading candidates Maggie 3.5-3.6. Then going to Pre-amp tube in the winter. Next summer the amp.

Keep the comments and suggestions coming, I am not firm on anything yet!

I thought it was interesting you happened to mention Maggies as a potential speaker choice as I couldn't think of a better example of why you should pick speakers first from a practical system building perspective. Ribbon speakers(which I believe the Maggies are--not electrostatic like Martin Logans) can present a very tough load for an amp, and if the amp isn't up to the task you'll never get the magic the Maggies can produce. It would be a shame if you flipped over the Maggies and found out the amp you just bought isn't suitable for power-hungry ribbon speakers(a lot of amps aren't)--your choice would then be sell your amp or forget the Maggies, neither of which is an attractive option. You just inadvertently pointed out one of the major reasons to choose speakers first(also one of Zaikesman's points above). Best of luck and have fun.

Tim

Zaikesman...thanks for your great elucidation of what I was clumsily delineating.
KT88...OF COURSE it's GIGO, but in absolute physical terms
(freq response, coherence, temporal/phase stuff) that are readily discerned, there is MUCH greater variability in the speaker-room coupling than in the sea of Redbook players.
And in the thread-head's case this is most apparent: his Nak CDP is in most ways much closer to supplying a musically-satisfying signal than his ADS boom-cha speakers are!
BowBow...the Acurus and ARC were recommended by NAD dealers as upgrades! Certainly the ARC sounded better, but the ACURUS was a gritty mess. The Alephs of course are another realm. I can't imagine that Lance's satisfaction with the ADS speakers will be improved with a more transparent amp!
Have you ever heard these speakers? And what if he buys a nice low-medium power high output impedence "sweety" and then tries it on the Maggies he just dropped on the thread?!
You really have to think of the speaker as a LOAD that must be properly DRIVEN. You don't build a car around a motor; you pick the motor appropriate to the load it must lug around, no?

Lance...if you try the Maggies then you WILL have to exercise care in setup and room-damping issues before you go preamp-hunting. I'm not sure I'd pick a tube pre next as the 2600 is colored enough to affect that decision too. As well impedence-matching issues may result in frequency response anomolies that could make your decision-making a bit complicated, and thus frustrating.
Get your speakers, then try to audition several pre/amp combos. If you're not sure, then at least try to use one you like long enough to audition several fine front ends.
The 2600 simply won't quite be transparent enough to make CDP comparos as easy as a great amp (and pre) will allow.
KT88's certainly correct here.
I'd focus on the amp chain as a WHOLE once you get the speakers selected. THEN get a real front end and your jaw will drop! Have fun.

ho hum.............. I'm getting bored. I bought my speakers last in my system, and it sounds wonderful!

All you who are saying if you were going to build a system you would start with the speakers are totally missing the point. The original poster just wants to do one upgrade; not completely rebuild the whole system.

Sure; if I always wanted a pair of Nautilus 802s, I would start with them and then choose the electronics. But if I already have a system and can only do one upgrade, what good is a great pair of speakers being driven by inferior electronics? It would sound like hell.