Recommendation for a highly resolving amp

I have Don Sachs 2 tube preamp driving Pass Labs X350.5 driving Sound Lab M545 ESL speakers. I like the sound very much, but when I went to hear my friend’s Spectral system (driving Quad2912 ESL) I realized my system can benefit from having a higher resolution amp. Spectral amps need all Spectral preamp/cabling, which would be quite expensive. What would you recommend?
@chungjh Are you streaming with Fibre into your Lumin X1. If not that would be the first thing I would do. Pretty simple and cheap to set this up with a Network switch that has Fibre outputs. This could make a big difference depending on the quality of your Ethernet setup.

Then try different amps.

it all starts in the front end but if its good next are the amps
my amp and preamp are Aesthetix very resolving

Enjoy the Music 
Can you explain exactly what 'resolving' is or means?
Not intended as bait; I really do not know what this means.
I have tried fiber optic and other top 6SN7 tubes. Not big difference. By resolving, I mean high resolution, clarity, detail.

What do people think about the Bryston cube series? Specs are similar to Spectral amps.