The consensus is that the voltage gain makes the biggest impact, with the input stage next, and the volume stage having the least impact.
My reference configuration is the following:
Voltage: SS2950
Input: SIL994
Volume: SS2950
Frankly, with the SS2590 and SIL994 in the voltage and volume stages, it doesn't really matter what you put into the volume stage. So, I wouldn't recommend getting a second pair of SS2590 (most people don't), but since I have them, I use them.
Others have put the SS2590 in the input stage with desirable effects. It boils down to personal preference and what other components you have in your chain.
I have found that the V7 Vivids are similar to the SS2590, so if you don't want to deal with extenders and figuring out how to make the giant SS2590 fit into your amp, the V7 Vivids are a great alternative.
The SIL994 is a great complement to either, but the V7 Vivid's dynamics don't reach the lows (mid-bass and sub-bass) like the SS2950, so it can be perceived as slightly more bright. This means that the SIL994 combined with the V7 Vivid might be a little too bright because the SIL994 is not really a warm op amp; it has good dynamics and great bass impact, but I think the mid-bass is slightly recessed.
A good combination for me has been the V7 Vivid in the voltage and the V7 Classic in the input, and either the SS2590 or Muzg in the volume. YMMV.
Everyone's experiences have been documented ad nauseum in this thread, including mine.