I felt like I should add two things to this for future searchers...
Bass Issues - I thought I was having these as well, and all of them ended up being something I had done. The two main ones were related to me using multiple 12au7s that I assumed one of which had to be good, along with the same issue with power tubes. I had a set of Mullard XF3 EL84s, the best EL84s made, which are known for having a very long life. They worked and biased up okay, but sounded thin. I had also bought a lot of 12au7 tubes listed as untested on ebay, which I think were actually rejects. Don't be cheap. Buy at least some decent tubes so you have a frame of reference.
My other issue was in my Klipsch speakers. I damaged a tweeter when I was working on the finish, so I replaced them with some supposedly factory replacements from a third party vendor.
They were either more efficient, or just screechy and annoying, which made me think the st-70 was light on bass. My other amp rolled off the treble so much that the tweeter wasn't noticeable.
A correctly biased VTA ST70 is a fantastic sounding amp with plenty of bass (I listen to everything from Coltrane to the Beatles to Rage Against the Machine to Black Sabbath to George Jones). It is capable of shaking my house at 105db through my Klipsch Chorus II's without breaking a sweat.
When you put in new tubes and bias them, the bias will drift as the tube gets older. This is why you should buy tubes that are matched AFTER a burn in period.
When I set bias, I set it as soon as I turn the amp on so I dont blow anything up, then I play it for awhile (you can hear if there is an issue) and check it again after about 20 minutes, and then again after an hour.
I check my st70 about once a month and adjust the bias to around 450 mv on my DMM, which I think translates to 45ma, but I am not sure of that...