You might want to add Line Magnetic LM 518ia or LM 508ia or LM 219ia to your list. Great value for the money.
We just helped a customer retube a Cary SLI-80, so we have to dig deep into the amp's design. Most of our experience with Cary before this amp was preamps. Mark at tech support was super insightful. The amp is amazingly well designed and versatile - excellent audio path 6922/6SN7/KT-88s but Cary overdesigned the power supply (a very good thing) so it can accept KT-120s and 5AR4s. Only 1 cap in the audio path! It acts as an integrated and even comes with a remote! Highly recommended,
If you have never considered headphones, it might be time to buy a Bottleneck Crack w/Speedball upgrade or a Darkvoice 336. Both are available for under $500 and sound great with a pair of AKG K7xx (still available for $199 on Massdrop) or a good used pair of Sennheiser HD600, HD650 or HD700s for under $500. I believe you will be surprised what you are not hearing and you can easily roll the tubes until your ears are content.
PS - I own a top-end Meridian system with an Oppo 205 that I seldom listen to anymore in favor of the headphones.
If you want the BEST, why don't you give Berning a call and order his one-off push-pull 6B4G, pure class A. This is the amp he uses in his rig with the Merlins. If you want more power, he'll build the same amp but with 300Bs. You can order it as a "control" amplifier with a 47 step attenuator for the ultimate performance (eliminate a preamp) if you only have a single source. He builds these himself in a prototype chassis - an upgraded anodized aluminum chassis is optional.
Well, I’m going to go against the majority here and say that for smaller speakers with under 90 dB sensitivity, I personally prefer solid-state amps... especially those that are biased heavily into Class A. Great results have been reported for your Merlins with both tube and solid state amps, but if it were me, a Pass Labs or Sugden integrated would be at the top of my list. The Positive Feedback review of your speakers mention that they achieved the best results with Pass Labs mono-blocks, which does not surprise me. Good luck and let us know what you decide.