Hey Dan, so now I understand what you want to do (I think so correct me if I'm wrong). But first a question, do you want to run 2 stereo preamps ? as this makes a difference to where you are thinking. Do you want one side of the ENGL to sound different to the other (Carvin TS100 accommodates this BTW) ?? or just use the ENGL as the "stereo" poweramp and have 2 different preamps to pick between ? Which is how I'd do it, then the best solution IMO is a small desk, plug both preamps in in stereo, use the aux/FX sends to get some FX on (either of them) and use midi patches to pick between different combinations (on/off/appropriate FX), feed the ENGL from the desk/mixer in stereo, heaven on a stick...IMHO. In which case you'd go with the matched Mullard 6L6s in the ENGL (since you have 2 already (but as SC says if they fit !!)). The TADs SC suggested are also great tubes (I very much respect SCs tube suggestions, partly why I found/fell in love with Mullard Long plates for my MP2 BTW), if the Mullards don't fit though, maybe get a matched 4 of the TADs ??.