Jsujo, The 63's have an impedence drop which can be difficult for some amps to handle (I think its down about 3.5 ohms in the bass area). Something to take into consideration in selecting an amp. FWIW I drove mine with a 40wt PP tube amp w/out any problems.
As to stacking 63's. IMHO, stacking is counter intuitive. 63's are point source speakers. If you stack them you will have two point sources to deal with. Try before you buy - YMMV.
As to stacking 63's. IMHO, stacking is counter intuitive. 63's are point source speakers. If you stack them you will have two point sources to deal with. Try before you buy - YMMV.