I am looking at an amp from a well respected designer that has a dealer network and the 125 watt amp sells for $3000.
My question is which Van Alstine (factory direct) amp should I compare the $3000 amp to, the 350exr, 550exr, or the 440 exh ?
Hard to answer unless we actually know which amp you're comparing it to.
If you look at Frank's website you'll find, under the products --> amplifiers tabs, a comparison grid. From that you'll see that Frank makes both ss and tube amps, and he builds an especially robust version of the ss amp, with the "h" designation, to drive low resistance loads.
I don't think that the 440 EXH, which puts out 220 WPC and will drive 1 ohm loads, is comparable to most manufacturer's 125 WPC amps. The 240 EX or EXH is probably closest (if the amp is SS). However, I can say from personal experience that the 350 EXR is a lovely amp and is very likely to outshine any $3,000 amp out there.