nothing you're doing or suggesting is a good idea...
for so many reasons. You should go to your local library and get some basic books on audio electronics, sound reinforcement and maybe something on acoustics (you might just be standing too close to the speakers in a confined space. Or google it, but there's a lot of misinformation on the internet.
you can probably use that 75 watt solid state amp as a PA wedge if you are very careful with positioning and you slave it off a dedicated mic preamp and don't try to run a low impedance microphone into a high impedance guitar preamp....
just, dude, get a book or two, read up, figure it all out for yourself
and don't buy anything else until you understand wattage versus sound pressure, impedances, etc etc... while you are learning you will have time to save more money to buy a better class of equipment with all the knowledge you have acquired
EDIT
In closing, you CAN do whatever you want with your amplifiers and assuming their output impedance guidelines are properly respected you will not break them... but there is no guarantee that a massively high wattage solid state amp will cut better over your drummer than, say, a low wattage tube amp (especially if you aren't setting either amp to cut through) and that a repurposed solid state combo will help you hear the vocals. Wattage is a power potential, not a measure of loudness, and even measuring sound pressure can be deceptive because the human ear hears different frequency ranges with different sensitivity and the confusion is further compounded by the fact that your ear also hears loud and quiet sounds very differently. Things will get even hairier if the 75watt amp is an open backed combo. There is a lot too this beyond "turn it all up." Although if you have enough Marshall Superleads and SVTs turn it up can be fun. With more modern consumer guitar and bass gear YMMV though.
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