I'm not talented enough to disable bands (nor do have the courage to mess around with things like that!) but I had noticed that my Venn Mobile speeds improved substantially after the Sprint-TMo merger. In fact, back in April, the phone showed "Roaming" notifications. I assumed it was roaming on TMo.
I could never (edit *) get tmo when I was on sprint postpaid recently. Could never force it either. Apparently they will be switching people over to tmo primary in the coming months. Doesn't seem to be any particular order who gets to switch first.
*I did roam on tmobile for 30 seconds in the middle of the night once actually. Shame really as tmo is 100x faster where I am.