That will work. The output might be a little hot (loud) depending on the gain of the amp that drives the speakers, but that just means using a lower setting on the volume control. One possible problem you may or may not run into is sometimes the volume controls are not as accurate at the lower settings, due to irregularities in the manufacturing, causing one channel to be a bit louder then the other, throwing the balance off. But you may not run into that, and it can be corrected, if it happens.
I've done that. It works. You still have enough "fine" control with the volume knob, though you do work on the low end of the range. Go for it. It sounds pretty good, waaaayyyy better than passive.
Of course, you'd be better off spending $150 on a stock Bottlehead Foreplay kit. You'll learn a lot, and both your MG Head and your speaker amps will sound better for being fed the Foreplay signal. With what you learn on the Foreplay, you'll be able to follow up with a number of mods to your stock MG Head, too.
But that's just my opinion. You may hear something else with your ears.