The sweet spot and beautiful music from the Mid Monos is the Tesla EL34s, as Mike already recommends. MIke and I talked about it a while back, and those amps were originally designed by him for those EL34s, circuit, plate voltage and all.
My buddy runs the Mid Monos with nice EL34 magic, for warm, emotional, holographic mids, more of the vintage tube sound. Just gotta love it. I run QS Mono 120s, specifically designed for KT120 / KT150s with higher plate voltage - and we compare. Each of us run similar efficiency speakers yet different sized rooms which matters too - along with the type of sound and control you are looking for from your particular speakers. Is more woofer control desired or high volume?
The later model mid-monos do support up to KT120 and KT150s too. Check your original owners manual if you have it on hand. If you are not sure and it's a much older Mid Mono version, Mike can tell you if the transformers will hang and not get too hot. Give him the serial #, for those amps, i.e. are they older screw-speaker terminals -or- the later generation brass-bannana speaker binding posts - ?
KT88s will give you more neutral presentation, still musical, yet bigger and cleaner sound, with more refinement yet you'll lose some of that more forward EL34 midrange magic, tone, voicing. You'll notice how I keep mentioning this part.
KT120s will have a little more deep bass grip and slam with authority on the woofer drivers, however you are leaning into the more analytical and neutral sound. Again further away from the EL34 magic in those particular amps. I believe the later model MMs are 475 plate voltage optmized for those EL34s, and not the upper 650v plate voltage like Mono 120s have, optmized for the bigger KT 120/150 tubes. What I've concluded about the QS amps and my formers is runing the tubes in their optimimum operating window. Mike really knows how his amps run each power tube.
If it were me and I wanted a slight change, and extra set of power tubes for the Mid-Monos, I'd opt for the new re-issue Gold Lyon KT77s as I'm more of a huge fan of that EL34 sound. KT77s are kinda like a bigger EL34 and retain a lot of that EL34 magic. Like others here, you might just end up swapping back to EL34s too, fwiw.
The last point is your speakers - what brand/model, ohms/impedance, db sensitivity at 1w/1m? And, what are you looking to try or change about the sound, if you can share more? This added info might help to steer you in a different direction, if that's what you truly want to change or to try something different from Telsla EL34s.
