I agree with Joe, PS Audio or CJ... ARC would not be out of the picture either in my view... but at this point the STP-SE is working very nicely for me!
And yes, I would think that the two amps would be very similar sounding. The features, many special parts being used and talked about, specs, SE inputs, heat sink designs, power, etc., etc. are similar if not exactly the same. I also assume both are designed by Nelson and the F6 was completed and in the market first. On several occasions it has been written by Nelson and others that design developed and optimized in the First Watt line get passed on to Pass Labs. So yea, I feel that they would sound extremely similar...
By the way, here is Nelson's description of the F6 to me:
"I found that it had many of the charms of the
other amps, but a nicely soft top end that forgives a lot of the top
end artifacts you get in today’s recordings."