power tube rating

I have a pair of VTL MB-450 amps, each with 8 6550 tubes. They were retubed in June of this year and now they need total replacing again. I was told that the tubes burned out this fast because they had a low IP value so I'm looking for someone to tell me what this value means, and what it actually measures. The tubes had a value of high 40's - low 50's and it's supposed to be high 60s. I need to know what this value is just so the next tubes I buy will actually last me longer.

Another question regarding tubes. The stock tubes are Svetlana, but does anyone recommend trying something else, maybe the Tung-Sol, SED or EH? Maybe KT-88s? I found the stock tubes to be "hard" sounding, so I'd like something warmer but with a full bottom-end.
Ip = Plate current. You obviously have the wrong tubes. VTL can tell you exactly what tubes you need. You would be money ahead buying your tubes from VTL. Changing the driver tubes to Mullard "CV" tubes will give you the warmer sound you are looking for.
The 6550 tubes seem to be correct for your amp. Was the bias set properly when the tubes were installed? How often did you check the tube bias? The bias can drift as tubes age. If the bias is not set properly the tubes can be run much harder than they should.
This is your first system? Where did you buy the tubes that only lasted 5 months. The answer to this question may tell us a lot.

If tubes are not matched properly the weak tube will place more load on the strong tube wearing the strong tube down quickly. This will also cause a rise in distortion which could cause the hardness in the sound you mentioned.

I have owned several VTL amplifiers and never heard the hard sound you refer to.
If you buy your tubes from VTL, they will ask what power tubes you want 6550 or KT-88. I prefer KT-88’s in my VTL amps. They are warmer on top and have nice punch on the bottom when compared to the 6550’s.