Well, this may be only speculation but I am guessing these are Chinese sourced and made to a price point (very inexpensive price point). Most internal components can be upgraded but if the circuit is just 'thrown together', there is not much more you will get from the amp. More expensive components may "change" the sound but not make it better. The transformers are another matter all together. As mentioned by Scott, if they are not up to par, the whole thing is a waste of time.
We all know China is capable of producing some decent stuff but when you figure in the profit marging PE has to make, one can assume these are very inexpensive pieces.
I use to inport products and know how the factories work. You can order 'bottom of the barrel" or top shelf quality. Most importers would order the bottom of the barrel pieces so they can buy more pieces, make some quick cash and move on. I am a bit sour now as I was burned badly due to transformer problems in a decent size shipment. I wanted the best quality transformers only to find out they were not suitable for US voltages and I had to throw away EVERY mains transformer. Thousands of dollars down teh drain.
For more information, feel free to read "Buyer Beware" through the link below.
http://www.responseaudio.com/AboutResponse.htm This is not to say that the PE amps will now be reliable or even sound quite nice for the money but I'm out.
Again, this is all speculation until more information comes out by users.
