In the past few days I've been looking at 15" and 18" speaker cabs to put under my 4x10 cab. I already know the brand that I'm going to get and it's Carvin. Heck I even know which cab, even though I play a 5 string, I'm still going to go with the 15". I think they sound a little clear and punchier. Every 18" that I've tried sounds muddy and lacks the punchiness that I like. The freq responses between the 2 cabs are'nt even that much of a difference. I just thought this would be and interesting post, and am very curious to know how everyone else here feels about the 2 different size cabs. I look forward to reading what everyone has to say.