Prices of the Ultra 350 and 550 have not changed from the previous verions, even with the circuit and cosmetic improvements.
Fet Valve Ultra 350 amplifier is $1849
Fet Valve Ultra 550 amplifier is $2199
Note that if you purchase the "little" one and wish you had gone for the bigger 550 model, you can, within a reasonable amount of time, send it (the new 350) back to be converted to a 550 for just the difference in price, plus $38 shipping of course.
Frank Van Alstine