I'll weight in here with an opinion. Can't guarranty the spelling or grammer though : )
In sum I'm going to strongly agree with chazboo's very nice post. It's really not a question of tube or solid state. It's a question of what speaker you are driving and which amp makes the best match for it in your ear's opinion.
A story: I am a Quad devotee. Loved the 63's since '83. Found them to mate very very well with Futterman OTL's (150 amps of big tube power as modified by George Kaye and NYAL). A match made in heaven. I used tube and solid state preamps. For years I used a Levinson ref 32. Nice system.
But quads, like any speaker, has limitations. I wanted more (who doesn't) sound pressure, more bass. So I explored big quads (989s) and wound up with a wonderful sounding stacked arrangement made by a fellow in Montreal (thanks Stew!). At around the same time I auditioned a pair of (ss) Halcro 58s in my system (good reviews and low distortion specs peaked my interest) Wonder of wonders the Halcro outperformed the Futtermans (IMHO) in my system. So I got 'em but could not part with my Futterman's. Now it happened that I had to get one of my big quads repaired so I pulled out my old pair of '63 for a while. Still sounded nice (but not in the same league as the stacked quads). For a lark I put the Futtermans back in for some futher comparison. Jaw Drop! The Futterman's gave the Halcro's a real run and in many respects sounded better. When I got my big guys back up no such luck: the Futtermans just didn't have it.
Moral to the story: what you are driving will make all the diff. The double stacked quads are a rather difficult load for the tubes. They got all congested and couldn't control the bottom end. On the relatively easier load of the standard '63s the tubes were magic. I was torn as to which was perferable. My (non audiophile) wife had no such reservations clearly favoring the '63s with the tubes.
Now I still have the tubes: they are in a second system I have driving LS3/5a's. And yes, a glorious sound. The Halcro's are driving the stacked quads as I speak in an all Halcro system. I am now going to try some tube phono sections and a tube cd player to see if they offer improvements to the overall sound of my system.
We shall see what we shall see!
