I'm with both blazer and spl on this on: I'd get a pair of 15's rather than an 18. Much easier to find in case you ever blow up the 18. And I'd go with multiple amps rather than a multi-channel amp if just for your bass (IF you have the room, that is). I bought a 4-channel amp to run my fronts and rears, but then before I even took it out of the box I changed my mind and decided to run seperate amps to fronts and rears, so I sold the 4-channel to my brother and got a new pair of seperate amps for door speakers and rear speakers. Several reasons for doing this, but the main reasons are that if some speakers quit working in your system, I've found it's MUCH easier to troubleshoot the problem if you run seperate amps. The other main reason is better sound quality and less stress on each amp (not to mention more power).
But I got a bit confused on your question. Are the 2 HiFonics amps you already have meant for your bass or are they for your other speakers? If they are JUST for your 18, I'd go with 1 big amp rather than 2 smaller ones. But if you get a pair of 15's to replace your one 18, I'd bridge an amp to each sub, then buy a seperate amp for fronts and rears or a 4-channel for those.
Just my opinion, which might not count.//content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif