The DT amps give you the ability to choose from the following power amp characteristics: 2 classes A/B and (quasi) class A - if you know how power amps work you'll know why I say "quasi" class A 4 different negative feedback loop topologies (tight, loose, zero or resonant) 2 different power amp valve operation modes (pentode or triode) In the DT50 amps you also have: 2 options with the 12AX7 boost (boosts the power through the extra 12AX7 valve that's there to give some extra push into the power amp for some models) 2 options with the B+ voltage on the phase inverter (high/low) 2 different feedback capacitors X/Y giving a choice of a tighter or smoother NFL So you have 2x4x2=16 different power amp characteristics with the DT25 and 2x4x2x2x2x2=128 different power amp characteristics with the DT50. And you can definitely hear the differences between those options. This ability to mix and match different power amp configurations with 30 different preamps and 18 different cabinet options that can all be preprogrammed into a patch on the DT500 delivers an unbelievable amount of tonal diversity. There's a lot more to in than just switching between class A and class A/B. That's why you should get a DT50 to go with your HD500 and not just a standard power amp. You'll have 128 different power amp configurations as opposed to 1. Cheers, Crusty