Just a bit of an update incase anyone is interested:
The house was empty last night so I took advantage and cranked the SLO a bit more, think I got it upto about 2.5 on the master, and it gets brighter as the amp gets louder. I turned the Treble and Presence down to about 3 and that more or less tamed it but there was still a little bit of shrillness if I played high up on the neck.
I tried the amp through my 1x12 with well worn in Celestion Classic Lead 80 speaker and really liked it. It was bright, but didn't have the shrillness to it that the sound had when through my larger 2x12 with V30s. So either my V30s aren't played in yet, which could well be the case, or they're just not right for my amps.
I'm now starting to think that a pair of V30s aren't the best fit for either this amp or my Splawn Quick Rod, so I'm looking into something I can replace 1 of the V30s with. Any suggestions?