I went from a W4S STI-500 (some of the same circuitry as the STP-SE) to a BAT VK-52SE with Hypex NCore's for amps. IMHO, I strongly disagree with the description of BAT being labelled as overly DARK or a WARM LAYER OF CARAMEL.
To be perfectly honest though, I think that for the price differential over the STP-SE, you are going to only get a small improvement in performance with ANY pre-amp.
Mine sounds perfectly neutral. I cannot tell how much of the change is coming from the new amps and how much is coming from the new Pre-amps, but overall, my system went from sounding excellent to superb.
I think the only way you are going to find out is to try one in your system. I think if you find a good deal on a used BAT or ARC unit that fits your needs, go ahead and get it and if it doesn't work out for you, you should be able to resell it for a similar price meaning you will only be out shipping costs.
Another one to consider would be the Modwright. I had it narrowed down to that one and the BAT and found a better deal on the BAT before the Modwright. I am perfectly happy with the sound of my system now and am not planning on upgrading/changing anything currently.