I have had a Mesa Boogie Mark 3 for nearly ten years (1986 model) and have gigged it every weekend since then. No problems with reliability apart from changing pre amp tube on 1st gain stage every few months.

I found that Mesa tubes are not the best to buy as they are made in eastern Europe and are basically bog standard tubes with a Mesa label on them. I got in touch with a specialist tubes retailer and bought valves made by Phillips instead. Huge difference in tone, relaibility and less noise too. Have been using the same preamp valve for 2 years at no extra cost.