Modest Preamp Upgrade?

I would stick with what you know well works in your system for a preamp. I happen to be a big fan of the 6SN7 based preamps but don't know the TAD sound. In sum don't fix what ain't broke.
So looking at your other ideas-
The Dynavector cartridge is very detailed and I think sounds great.
Cables too many variables but the signal cable ICs I own are not in use . You can do lot better but I have no idea what you hope to acheive.
The EH6CA7 is one of the biggest sounding of the EL 34s if you put aside the KT77s for a minute. I like JJ E34ls but they don't synergize with my monoblock tube amps which came stock with the EH 6CA7, not EH EL34. I hope in you mean 6CA7 specifically as well.
The only thing I was thinking of changing to was the GL reissue KT-77s. The JJ KT-77 sound great but one blowing after 10 minutes scared me off. The buzz on them has been notoriously unreliable.
The GLs supposedly sound even better but they haven't been out long enough to comment on reliability. They also cost far too much for current production. They cost $20 more a piece than the GL KT-88s, themseles high priced already. I use the GL KT-88s in a different system. I must say they use they are fantastic tubes. I guess given the cost of NOS the nearly $50/tube I paid is OK.

Mechans,

I am not good with audiophile jargon and can really only tell if I like something when I hear it. Usually it is in small degrees. For instance, I was running JJ KT-77s in the TAD amp and I liked them ok but compared to the EH6A7s (yes specifically these) or the Cayin branded EL34s they sounded a bit thin to me (I can't tell much difference between the two EL34 variants mentioned). I have never been particularly fond of triode or class A amps which tend toward what I descibe as thiness or lack of robustness. I have had a Forte 4A, McCormack DNA-125, and a Classe CA-100 (still have in a second system). I prefered either the Mc or the Classe over the Forte. In other words a good AB amp over an A just sounds better to me.

On the other hand, I love the quiteness and detail the Sutherland lends my system. I guess what I am trying to say I really have to strike a balance between detail and robustness for it to sound right to me.

The Signal Cables may be a weak link but I am not sure where to go. All I can tell you is they were a big improvement over anything else I previously had in my system which were relatively inexpensive Tara Labs and AQ interconnects.

The Dynaudios probably could use some more power but amazingly in my relatively small room 60 tube watts do pretty well listening to mostly jazz. I do miss some slam, quickness and headroom when I listen to 70's rock however.

Thanks for your thoughts,

Signal cables are quite well made and spec'd from what I can tell. A little up the price scale and worth considering are the cables from VH Audio, but my impression of the Signal stuff was that it was pretty good.