I can't speak for everyone but I have had mine for over a year now and never experienced a cutting out issue on several gigs and rehearsals. I think the issues have been ironed out and folks who had the issues got them taken care of by Ampeg. I wouldn't worry about it.
Regarding the tube, preamp tubes are less of an issue to change than power tubes. You don't have to re-bias the amp for different preamp tubes. Not a big deal. However, preamp tubes last much much longer than power tubes, so you won't need to change it for a few years at least. If you want to change it for a different response that's fine, just swap it for something else. No other steps needed. FWIW, I am still using the stock tube and the amp sounds great.
Don't overthink this. If you like the tone of Ampegs and the power of the 7Pro meets your requirements, then grab one. Their customer support is very good and the problems with reliability seem like a distant memory at this point. I love mine and have no issues recommending them.
Don't think that JimmyM would have said it better.. right on the mark!!!!!

