FWIW, the line section of the MP-3 has 4 tubes. The MP-1 line section has 6 tubes. Either way, the tube warranty is for a year.
If you are one who is into swapping tubes, then I recommend you run the unit without the cover until you have sorted out which tubes you want in the unit, and use cables that are long enough to allow the chassis to be moved for easy access, thus avoiding Tvad's objections. Longer cables are not an issue with either preamp- the MP-3 can easily drive 50 feet so an extra foot or so behind the unit can guarantee easy access.
Both units, like any tube preamp, make some heat so we recommend (in the manual) that if the unit is in a cabinet, that there be at least 2" above it. This will also make it easier to deal with if you have to pull it out for a tube replacement. In either line section, the tubes are operated well below any of their maximums- we've seen tubes in the line section of the MP-1 go over 50,000 hours. We don't have good data like that on the MP-3, but we have seen the tubes in them last over 7 years with lots of use, so in a nutshell, the line sections don't eat tubes and don't have a reputation for it either.
If you are one who is into swapping tubes, then I recommend you run the unit without the cover until you have sorted out which tubes you want in the unit, and use cables that are long enough to allow the chassis to be moved for easy access, thus avoiding Tvad's objections. Longer cables are not an issue with either preamp- the MP-3 can easily drive 50 feet so an extra foot or so behind the unit can guarantee easy access.
Both units, like any tube preamp, make some heat so we recommend (in the manual) that if the unit is in a cabinet, that there be at least 2" above it. This will also make it easier to deal with if you have to pull it out for a tube replacement. In either line section, the tubes are operated well below any of their maximums- we've seen tubes in the line section of the MP-1 go over 50,000 hours. We don't have good data like that on the MP-3, but we have seen the tubes in them last over 7 years with lots of use, so in a nutshell, the line sections don't eat tubes and don't have a reputation for it either.