While my situation is not identical to yours gear-wise, it may be close enough to be somewhat meaningful.
For the last week and a half I've been enjoying a new-to-me used Symphonic Line "The Enlightenment" tubed pre-amp. It is currently feeding a Plinius SB-301 driving Revel Salon 2s. It was rebuilt soup to nuts within the past 18 months by an authorized SL technician using the same parts that go into the newly built units. So it is in essence a new unit sitting in a 1999 chassis.
The SL pre is replacing a Plinius M8 pre. The M8 provided strong weighty bass, good dynamics, dark background and a broad soundstage, and mated quite well with the SB-301. The SL pre is providing just as much bass weight with more detail, a plainly superior mid-range, and a far more detailed treble that at the same time is less fatiguing at higher volume. The SL pre also is plainly better in terms of micro-dynamics and depth of soundstage. I would characterize it as just slightly on the warm side of neutral, but not at all rolled off in the treble like some other tube pre-amps I've heard. Nor is it at all slow - this sucker is fast on the attack.
This is not to say that the M8 is a lousy pre-amp. It's actually a quite nice pre-amp and provides a very solid sound. But the SL pre is leagues better and frankly has showcased the SB-301's talents in a whole new light.*
Hope this is somewhat helpful, and good luck finding the improvement you seek!
*Alas, the SB-301 faces heavy competition this week from a pair of incoming Symphonic Line Kraft 250 monoblocks.
