Not sure about your price-range but VAC is a thought. I run a Phi Beta pre-amp into Bryston 28B-SST2 amps and the combo works beautifully together. This VAC model, like the Signature 2a, is based on the 8416 tube, an Amperex 12v version of the 6DJ8/6922/7308 family. But it has a switch enabling the use of the latter, so you can tube-role to your heart's content. I prefer the 8416, personally.
The 28B input impedance is 16.5K, single-ended, with 23db of gain on its low setting, which I use. Not sure but I'd think the 4B-SST2 would be the same. VAC claims the Phi Beta will easily drive loads down to 200 ohms, so no issues there; and it outputs 12db of gain. VAC pre-amps other than the Phi Beta and Sig 2a are much higher gain--more than 20db, I believe--so be sure to check with VAC about compatibility if you're thinking about trying one. They're helpful folks.
The 28B input impedance is 16.5K, single-ended, with 23db of gain on its low setting, which I use. Not sure but I'd think the 4B-SST2 would be the same. VAC claims the Phi Beta will easily drive loads down to 200 ohms, so no issues there; and it outputs 12db of gain. VAC pre-amps other than the Phi Beta and Sig 2a are much higher gain--more than 20db, I believe--so be sure to check with VAC about compatibility if you're thinking about trying one. They're helpful folks.