Best option for Amp upgrade...

shoff,

I suggest you sell your McCormack DNA-500, especially if you think you can get $3k for it. It’s likely got a few years on it and certain internal components don’t last indefinitely, especially power supply capacitors.

Your Rockport Mira II speakers are very good spkrs that are not very efficient that usually sound their best with a powerful amp or amps driving them.
I think a pair of D-Sonic M3-1500-M class D monoblocks would be a very good match for your speakers and CJ preamp. Plenty of clean, undistorted power to drive your somewhat inefficient speakers with enough in reserve for increased dynamic range. Here’s a 6Moons review:

www.6moons.com/audioreviews/dsonic/1.html

Because these amps are so neutral in nature and add little sound of their own, the sound characteristics of your tubed CJ preamp will be the main factor in the flavor of your system’s sound. Good news if you like the sound of your preamp.
These amps are based on newer Pascal class D power modules that combine the power module with a switch mode power supply (smps) on a single board. You’ll notice the review lists the amps as D-Sonic M2-1500-M models. D-Sonic’s owner, Dennis Deacon, decided to change the model name to M3-1500-M right after this review to signify the change in the internal power modules being used and their increase in performance.
There’s little risk in buying tese and trying them in your system since D-Sonic offers a 30-day in-home trial period with a "no questions asked" return policy. I doubt he has many refund requests.

Tim

     I do not doubt having the full SMc mod performed on your McCormack DNA 500 amp would likely improve its performance and that key vulnerable internal parts could be replaced at the same time.  But it is expensive and while the amp design may still be very good it is also now about 15 years old.

      I've never tried the DNA 500 in my system, but I am aware of this amp's excellent reputation of  being powerful and stable enough to drive virtually any speakers while also possessing a very detailed, refined and musical mid-range and treble.  A revolutionary design/amp that probably will continue to sound very good until it  is unable to continue due to some unexpected cause.

     The main reason I thought of the D-Sonic M3-1500 class D monos as a good solution for your situation is because I think their performance and strengths are very similar to the DNA 500's performance and strengths: extremely powerful, very high damping factors for solid bass control, high reserves of power for live-like dynamics along with a very detailed, refined mid-range and treble response that is never excessively bright or brittle.  

     However, the D-Sonic monos are based on very recent advances within a newer technology, class D amplification, which may scare off some overly risk averse or unadventurous individuals.  I just think you'll be pleasantly surprised how reasonably priced, reasonably sized, electrically efficient, cool running and great sounding these mono-blocks are.  
     I definitely would urge you to give them a 30-day audition in your system before spending considerably more on any alternate option.

Best wishes,
   Tim