Great thanks! Yeah I went with your suggestion awhile back and am using the 5687 on the OTL - great upgrade. I think I'm going to try Telefunken EL84s. I'll look for the 6P15P you mentioned as well.Ha! I saw that and was close to pulling the trigger myself (great deal! Congrats!!). I had that in my alerts, but when the moment came I realize the I’ve tubed my MK2.5 (which uses 12au7 gain tube, it changed with MK3) and also has single vol knob (the change to double vol knob I find mildly annoying though no biggy). So I exhaled, and decided to be content… happy this went to a good community member!
I don’t think you can use 5687 instead of EL84 - but you should try the 5687 in the OTL (perhaps you have already, I forget, sorry). There are ton of great EL84 options that are fairly easy to source. I settled on the Reflektor 6P15P which is an equivalent that can be rolled in (also tried the RCA 6BQ5 EL84 that worked great)
The combo is just terrific, you’ll have a blast fine tuning it!
Yeah the dual volume knob is a bit of a bummer, but hey it's a pretty new MK IV and I'm thrilled just to have one. How big of a deal is it to use the OTL pre's volume control instead?