Upgrade from Martin Logan Sequel II's to ????????

I do not believe the Logan’s are your weakest link.  I believe the weakest link is the internal MAC DAC and use of a PC for streaming.  I recommend a dedicated DAC  and streamer.   There are many choices each with their own sound pallet.  Go with preowned units.  For example, a preowned Bticasti M3 and Aurender N150 will get you in or slightly over budget based on HifiShark recent sales.  
 

if you are set on speakers, +1 @soix .   Define what improvements you wish to achieve over the current SQ of your system.  Once you have a goal, there may be options to reaching your goal other than disowning the Logans.  

@jsalerno277 +1. IMHO your streamer, DAC, and phono pre could all be significantly upgraded. 

Since you like panels, look at these

https://www.audiogon.com/listings/lisbecib-martin-logan-montis-floorstanding-speakers-dark-che-77931-full-range

Although they're a couple generations old now, a step ahead comparing your ML's

Still relavant and better mids/highs than many current $10K speakers?

Long game-If you're a real LP fan, separate phono and upgrade from the AT OC9 to ART9(big upgrade for not much $)

The LFT-8B should fit the bill!

https://www.eminent-tech.com/main.html