I would definitely go for Art Audio Quartet monoblocks. They drive my friend Andra Eggleston,without any problem at all.They are so musical,they posses 3D layering,and the vocal makes you feel, the performer is in the room.
Audio Research Tube Mono Blocs such as the VTM 200 or the VM 220's would be a phenomenal match with the Vandersteen 5's. I've heard that combo a number of times... exquisite! well within your budget (used) as well.
As an added bonus, they'll also do a great job with your Martin Logans as you prepare to do the speaker upgrade.
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