the general feeling is that if you have a 100w amp and you max the volume you should get a 200w rated attenuator right? 50w amp =100w attenuator?......

well i have a jcm 900 100w that comes with the half power switch. i use it at half power cos i have a weber mass 100w attenuator. doing this im told (using 2 output valves/tubes instead of the 4) will shorten the valve life. got my amp revalved 2 months ago and a valve blew yesterday.

question is...would i get away with using the 100w attenuator with the amp on 100w mode if i kept the volume at say 7/10??
Nope. Most 100 watt Marshalls hit 100 watts around 5-6 on the (master) volume knob. Some sooner than that.

Tell me more about this half power setup you're using. There's no good reason that a proper half-power circuit should shorten the life of your tubes by itself. If you're cranking the tubes hard because of the half power, or the attenuator, that's a different story. What does the switch do? Triode/pentode? Did you actually pull out 2 of the 4 tubes, or are they still in there? Was this aftermarket or did it come that way from the factory?
marshall jcm 900 100w 2x12 combo. has 3 x 12ax7s and 4 x 5881s. there is a half power switch that changes it from 100w to 50 watt. i just know it uses 2 5881s instead of the 4. i just crank it cos i like the full sound of the overdriven 5881s. have it in 50w mode cos i have a 100w attenuator. kinda pissed that im going through tubes so quickly.....only use the amp once a week for 3 hours.
The stock half power switch on the 900 does not turn off two of the tubes. Unless you have an aftermarket modification, it's a pentode/triode switch.

If your amp was biased too hot or had some impedance mismatch due to an odd 1/2 power switch setup, that could cause you to blow tubes early if you were cranking the amp up very high, whether it was into an attenuator or not.

My first line of defense here would be to get a tech to check the voltages and bias, and toss a quad of 6L6GCs in instead of 5881s. The GC is a much tougher tube and should last longer if you're pushing them hard. Alternatively, you could run it in 100 watt mode, pull two tubes, and change the impedance from 8 to 4 (or half of whatever you're currently using) so that you only burn through a pair of tubes at a time instead of all 4. That gives you 50 watts of output without using the triode switch. That seems like an unlikely cause for your issue, but if you stop blowing tubes doing that, you've at least got a diagnosis.
I'm not a big fan of triode/pentode switches in general, but they're not necessarily the worst option. You're going to have to knock down the volume somehow, and any way you do it is going to mess with your tone.
The fact that you're using a half power switch and an attenuator tells me that you're either squashing the signal quite a bit, or you're still playing quite loud. Have you tried going back to the simplest setup and adding options? That is, amp in 100 watt mode, crank the preamp, keep the master volume down to control the output? Then add triode mode, then the attenuator (only if necessary), using the attenuator as little as possible until you find the right tone? The idea is to keep the extra options out unless you're absolutely sure you need them.
thing is i find the amp thin and tinny at low master volumes. i dont play with alot of gain (think cranked plexi kinda gain) but i have to have the seperate clean channel. otherwise i would get a marshall bluesbreaker amp i think. typical settings for me on the overdrive channel are:
preamp gain 3/10 master 8/10. turning the preamp up much higher sounds too mushy
OK, fair enough. Just making sure you weren't missing any cheap or easy options. It sounds like you're using it effectively, so the question now is why you're blowing power tubes. A trip to a tech to check your bias and voltages would be a good idea. Do you have a way to check your wall voltages, or access to a power conditioner?
im going to the tech on thursday. ill make sure he checks the voltages and bias. has just been to marshall 2 months ago so surely they wouldve checked it then...one more thing.....if i ran it in pentode with a 200w attenuator would i hear tonal differences? punchier, brighter?
i think i will experiment with it though.
I can't say for sure what a different attenuator would do. It would definitely depend on the type. A Weber MASS would probably get darker as you pushed it harder, where a resistor-based unit might do it less, or not at all. Punchier and brighter to me would involve using less attenuation and turning down the master volume.