New or Old?

depends on how old the amp is and how long you plan on keeping it...I've kept my big Rotel about 14 years now, glad it was less than a year old, not 10-15 years old when I bought it. It's really a matter of personal preference, I'm much less particular about home theater than my 2 channel system...If it was now, I would probably buy a used amp a couple of years old...

The old Krell Theater Amplifier Standard (TAS) was an awsome amp, but at this point it is 15-17 years old and I would not spend money on that unless you planned on re-capping the amplifier. The Krell devices from this era are starting to frequently fail - nothing bad here, it’s just the age of the device. There is a Krell S-1500 on audiogon for $2600 which is the newest amp that sounds close to TAS and can be up to 11 years old.
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The newest Krell amps, such as Chorus 7200, are very good, but they are full Class A and have a more smooth liquid lush type sound. Excellent if you want that sound, but not as clean and clear as the previous generation amps. I remember discussion that your ML Edge were somewhat forward sounding, so you might end up liking the Krell Chorus 7200 because it does have softer highs but it still has good 3D dimensionality. It is not going to be as warm or laid back as something like a Parasound or McIntosh.
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There is a 3 channel Ayre V-6Xe for $3204 that would be another good high resolution amp (but only 3 channels).
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You could go with Krell Chorus 7200 and a Krell Foundation processor if you want to keep the same "look" on your equipment.