ampllifier design

If you want to use EL84s as output tubes . . . they're a good candidate for traditional Ultralinear loading, operating class-AB1 with cathode bias - you'll get a solid ten watts with very respectable distortion.

Choice of driver tubes of course depends on the driver circuit topology - you could do it Dyna-style with a pentode driving a split-load phase inverter . . . look at a schematic for a ST-35.

Personally, although I'm a Dyna fan . . . I prefer a long-tailed-pair phase inverter, and it's really hard to improve on a well-implemented 12AX7 or 12AT7 in such an application. For an input tube, if you wanted high loop gain, then an EF86 pentode would work great, and what you end up with is basically a Mullard 5-10 circuit. But for a modern amp (especially if driven from an active preamp), then the gain is not as necessary, so I'd probably use a plain ol' 12AX7, and share it between the both channels. There's a good reason why the 12AX7 is so ubiquitous . . . if correctly implemented, it kicks butt, and it's a reasonable assumption that they will be easily available long after we're all dead.

No matter what you choose, I'm a big advocate of well-implemented feedback amplifiers, and with EL84s, there's no reason why you should need more than two driver stages, and the input stage should be DC coupled to the phase-splitter for minimum phase shift. If well designed, a traditional EL84 amplifier can be of reasonable size and price, good stability, long tube life, good speaker compatability, zero maintainance, low distortion, and very good sound quality.

I likewise have good associations with the 6CG7, it's the long-tailed-pair phase-inverter tube in my Marantz model 2s . . . it's used effectively for this role in the 5 & 8 as well, and as a cathode-follower in the 9.

But if you're using EL84s as outputs, then the 6CG7 is kinda overkill as a driver tube, and you might have a problem making it work at its best as a long-tailed-pair from the lower B+ plate supply (Marantz had a higher B+ available because he was using EL34s as outputs). And for an input tube, the 12AX7 is a better choice because of the higher mu.

I personally don't feel that different tubes themselves have any "sound" at all - it's the circuits that they're used in that have certain "sounds" . . . and it's just that some tubes are traditional choices for certain circuits, and thus we have these associations. I also think our associations of NOS tubes as "sounding better" is because their manufacturing tolerances were much, much more consistent, so it was/is possible to make more precise engineering decisions about critical circuit parameters. Added to this is the fact that most of our tube data is NOS as well.