Great suggestion, BGD.
My Peavey has a knob on the back called "TEXTURE", which is essentially a power soak (a.k.a. = attenuator). It enables a reduction of up to 60% in the signal path from the amp to the speakers, when rotated fully counter-clockwise. I checked the Marshall MA specs, and don't see anything like this mentioned.
All this said, it's still fun to get the gear into a bigger venue, and let it work to its full potential. Here's the text of an article on attenuators, from the "SoundOnSound" website:
Can a power soak really give you "loud tone" without volume?
Using a 'power soak', or speaker attenuator, allows you to turn up a guitar amplifier to the point where it produces its optimum distortion and compression, but then regulate the actual volume of the loudspeaker(s) it is driving. In the home studio, where volume levels may be restricted, this allows you to still record using valve (tube) output-stage distortion. There's no point in using a speaker attenuator with a solid-state amp. You might as well just turn the amp's master volume down, as there are no tonal advantages to running a solid-state amp at high volume. You might get some speaker compression and cone break-up, and you'll get some interaction with the guitar if you are in the same room as the speakers, but that's all.
To provide independent regulation of speaker volume, the power soak needs to be patched in between your amp and its speakers and must incorporate a 'dummy load', consisting of a number of large resistors, so that the power from the amplifier has got somewhere to go when it's not going into the speaker. Instead of being turned into sound by the motion of the speaker, it will be turned into heat and safely dissipated. If you run a solid-state amp with no 'load' (ie. no speaker connected), it will see it as an infinite impedance, which just reduces the amp's output to zero. Drive a valve amp into no load for even a few seconds and you could be looking at a very large repair bill.
Guitar TechnologyIt is very important for the health of the amplifier that the dummy load section of a power soak should have an adequate power rating and also replicate, as far as possible, the exact load that the amp was designed to drive. This is why power soaks often have one specific impedance that they are designed to work at — 8(omega), 16(omega) and so on. To operate at high levels without risking damage to the amplifier a power soak must also replicate the 'impedance curve' of a real speaker — loudspeakers do not present a constant load at all frequencies; the impedance rises at high frequencies and there is always a peak at the resonant frequency. Although some early designs of power soak were purely resistive and led to frequent amplifier breakdowns, any reputable modern design will now provide an appropriate 'reactive' load, like a speaker.
There is, in practice, a limit to how far you can attenuate the amp/speaker link and still have it produce a desirable quality of output. THD's renowned Hotplate attenuator (www.thdelectronics.com), which has one of the better reputations in this field, actually incorporates the speaker as part of the total load and its smaller attenuation settings — -4dB and -8dB — sound quite acceptable. As attenuation is increased, it becomes necessary to utilise the on-board EQ switches to boost lows and highs, then when you reach the full dummy-load setting which allows attenuation right down to zero, the sound becomes so limited at both frequency extremes that you probably wouldn't want to record it anyway. The Richter Control attenuator, from UK manufacturer Sequis (www.motherload.co.uk), utilises a different design that gives it a slightly more open sound at very high attenuation settings, but it can only pass a maximum of 50 percent of the input to the speaker.
It is possible to achieve really good recorded results with attenuators, but the key to making the most of them is to recognise the point at which they are doing more harm than good. A major part of the sound of a loud electric guitar comes from the electro-mechanical behaviour of the speaker and its interaction with the amplifier's output transformer. At very low levels, neither of these will be occurring in anything like the same way, so it is not really surprising that it doesn't sound the same. If you find yourself having to attenuate your amp beyond the point where it sounds good, try using a full dummy load setting and then take a DI signal from the attenuator out to a speaker simulator, in either hardware or software. This will often sound far more real than an over-attenuated speaker. Alternatively, feed the attenuator into a different speaker to the one you normally use. A small speaker driven hard will invariably sound much more authentic on distorted guitar than a large speaker barely working, and with the attenuator you can set the exact power level where cone break-up starts to occur. (Dave Lockwood)
ACOUSTICS: Cordoba D10-CE / LaPatrie "Concert" / Takamine GD30CE-12 / Norman ST30
ELECTRICS: EP Les Paul Custom Pro / Gretsch Streamliner G2420T
AMPS: Traynor YCS50 / Peavey VK212 / Traynor AM150T
EFFECTS: Boss ME-80 Multi-effects / Ibanez WD7 wah