Upgrade my Yaqin MC-100b for $2,000 - should I or not?

The scenario:  The Yaqin MC-100b is my first real tube amp.  I'm using it to drive the panels only on a set of Martin Logan SL3.  The woofers are bi-amped on my SS Nakamichi 100WPC old-school amp.

I LOVE THE SOUND TODAY.  I've rolled tubes in the Yaqin and my room sounds wonderful to me.

I listen a couple of evenings each week...so the amp is turned on and off lets say four to five times a week.

If you want all the details on my room/system, here it is:  http://www.martinloganowners.com/forum/showthread.php?17931-System-541-(SL3-Logos-Aerius-I)

Please don't give me a bunch of grief about the surround speaker placement...it's not gonna change...and I do have room treatment now.

What I want:  Real feedback about the potential of upgrading my Yaqin to a used Tube Amp.

I'm looking through the ads here and see plenty of options under $2K.  I am not going to spend more...I don't care if there is something at $4k that should be at $20k...I told myself I am ONLY going to spend my credit card rewards...not one dime more as I am pretty dang happy right now.

I feel like the Yaqin is hitting way above it's price tag.  Everyone that listens to my room is blown away.

So...would an Audio Research D-70 be a good option for an upgrade?  What about the Genesis Advanced Technologies M60 Mono's?  PrimaLuna ProLogue 7?  Rogue ST100?  Conrad Johnson LP-66s?

Thank you in advance to anyone that has feedback for me. 

The ML are said to be hard to drive as they have a pretty challenging impedance curve at the top end...but the little Yaqin seems to be doing fine.

-TooSlow