I think you are asking too much. The THR is what it is beacause all the options taken by Yamaha engineers, and it is include the pair of so tiny hp's. A THR with bigger hp would be just a bit too big to be comfortable to use as a plug and play low noise amp.
And if you plug it to a pair of 12', it may sound ok ... but if you push the master, will probably ear the same flaw as yuou eard with your active cabs, just because you can't expect much from a little transistor amp, as soon as you try to make it provide a bit more power. The THR is made for sounding ok at low db lvl and it does it very well, much better than any tubey will ever do at the same price.
If you want more headroom with clean or even crunch sound, then get another amp
On dit d'un fou qu'il a perdu le sens de la realité.
Alors que dire des gens qui confondent :
- La colere et la haine
- Le respect et la crainte
- L'offense et la peur
- La vérité et la croyance
- Le savoir et l'information
- L'amour et l'envie
- Etre et avoir