Remember that the Two Stroke circuit is very much in the style of a Black Face Fender era design and was not designed to offer much clean headroom when turned up.

You could try changing out the pre-amp tube to another of the 12A_7 family with lower gain. A solid state rectifier is easy to try and give a little tighter bass and slightly more mids, it’s louder too. You can try the 5881 or KT-66, they won’t be much cleaner thought. Do not try to use a KT-88.

If you really want an amp with clean, sparking highs that’s great for pedals, build a Spitfire clone. EL84 power tubes offer clear, sparkling highs and although the Spitfire can get crunchy with the pre-amp volume dimed, with the pre-amp turned down a little and the master volume up, the amp is very clean (and loud). It’s a great amp for pedals and 15 watts is very gig-able.