Okay I have this old Randall RG-180 is a 100 watt combo with 1 15" inside. But the speaker is cracked so I decided to get a new one. Now my question is should I just buy the 15" or get two 12"? What is the benefit of having two 12's over a 15? Also any suggestions on cheap speakers that would suit this amp?
You might not be able to fit 2 12" speakers in there. Just stick with the 15" ones and you'll be fine.
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You can't replace a 15 with 2x12s, it just doesn't work.

Get an Eminence Legend 105, excellent speaker and decently priced.
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