If you're using your Valvetronix, make sure you use 1 of the 3 OD models with every amp model you choose.....even clean ones. If you don't use any OD models, the amp does sound a bit sterile. Just keep the OD volume high and the gain lower, just to add some punch to the models.
Also, it's very possible that the stock speaker sucks. I use the head with a Bogner 2-12, so the amp has the benefit of real speakers.
Before you give up on it, maybe you should look into a new speaker.