I have a 1960's Vox 100 amp Royal Guardsman and a Vox Westminster cab with an 18" speaker. I'd like to add to this an additional cab with four 12" speakers. The head only has 1 output. I'd like to run both cabs off this one head, and have something that will direct the lower end frequencies to the 18" cab and higher frequencies to the 12" cab. Any suggestions? Preamp? Frequency splitter? Etc?