Cheap amp for Stax SRD7 and SR-X headphone

Dear all, greetings.

I have to confess that I enjoy listening to music but know very little about HiFi. Some time ago I happened to pick up a pair of Stax SR-X MK III 'earspeakers', which I use with a Yamaha RX-V467 AV amp (the family use it for TV and such). Although not what people consider good equipment, I quite like the sound of it. But the complexity of needing to re-connect wires and change settings from multi-channel to stereo and back is stopping me from using it everyday, so I set forth to buy an amplifier just for the headphone.

Initially I turned to a good friend who was into vintage tube amps, and he let me try out one of his 'good' amp with geniune vintage tubes. He even suggested that I could keep it at no cost, but after finding out that how much the tubes cost I have to give it back to him - it is too much a gift to accept; and frankly I don't like the sound of it. It is full of harmonics, very warm but muddy.

I then read about the T-amps on forums and tried out a few, since people say that the headphone don't need much power to drive - they were wrong. The 25W ones clearly didn't give enough sound and even the bigger ones (50W if I remember correct) sounds awlful when I turn volume up.

So could anyone suggest something which will work for under $2000? I don't mind buying used but when I check I found that stereo amps are very expensive, plus I will be concerned about realibility if it is too old.

Thanks in advance.