My room is nearly identical in size and I have tried most of the usual suspect integrateds (including the Naim and Rega you mention) in your price range...my favorites:
Naim Nait 5 (bargain! Can be had between $600 and $650)
Musical Fidelity A3 ($500 +/-)
Musical Fidelity A220 (no remote; rare - $400 to $500)
Byston B60 (non remote version - watch for it! sometimes <$700)
I could easily live with any of them long-term! All do a great job with monitors such as your Merlins.
All have a very different signature so audition is necessary. You should be able to purchase and resell with little to no loss if you you end up deciding that the sound of a given amp is not for you.
Good luck!
Naim Nait 5 (bargain! Can be had between $600 and $650)
Musical Fidelity A3 ($500 +/-)
Musical Fidelity A220 (no remote; rare - $400 to $500)
Byston B60 (non remote version - watch for it! sometimes <$700)
I could easily live with any of them long-term! All do a great job with monitors such as your Merlins.
All have a very different signature so audition is necessary. You should be able to purchase and resell with little to no loss if you you end up deciding that the sound of a given amp is not for you.
Good luck!