Some experienced VAC customers and VAC dealers will tell you that the Renaissance amps are the best amps made by VAC and that switching to the Phi amp will be a step backwards. VAC stopped making the Renaissance amps because customers were not happy about the high cost of retubing with 300B's (the Phi amps use cheap pentodes) and the high electrical draw resulting from the Renaissance amps' Class A biasing. In addition, production costs are lower with the use of circuit boards - the Phi amps use cheap circuit boards - instead of the expensive point-to-point wiring used on the Renaissance amps. Of VAC's current production amps, only the very expensive "Statement" amps feature point-to-point wiring.
You would gain a bit more power with the Phi amp, because, having approximately two times the wattage of the 70/70, you can expect to obtain approximately 3 db. of additional volume.
You should definitely audition the Phi amp in your system before purchasing one. I'm not saying that the Phi is a bad amp - VAC makes very good gear and provides fine service - only that you need to determine whether you already own the better amp.