I don't think a standard SRD-7 would be any better than the SRD-7 SB. The transformer with the Fisher amp would have good dynamics but less detail than a dedicated stat amp. I also found something like my previous SRM-1 Mk2 Pro to have a bigger more holographic soundstage.
So, a vintage Stax amp like SRM-1 Mk2 Pro or SRM-T1 would sound better on a budget, but they're all getting so old that you'd want to do a refurb on them (old caps and such). That's why I said just use what you have and enjoy it - I didn't know you would be able to build an amp. If you really wanted an upgrade without going vintage, and the budget was higher than an "SRD-7 level" budget you could consider a Woo GES or build a KGSS.
The GES offered an excellent pairing with my SR-Lambda. But my SRD-7 SB out of my Eddie Current ZDT sounded closer than you'd expect to the GES, with less detail and narrower soundstage but otherwise excellent sound from the SR-Lambda. Right now I don't have a working normal bias amp, so I use my SR-Lambda with an SRD-7 SB that was modified to have the input go directly into the transformers via banana jacks, and bypass the old stock input cable and other crap. This sounds a little more detailed and transparent than my stock SRD-7 Mk2 SB (one pro and one normal jack).