I've used a few 18's over the years with vastly different performance characteristics. I found myself using a JBL E155 for a while because it was cheap and came in a Mesa RR bass cabinet. It went subwoofer deep and handled a 300w tube amp with grace. It was every bit as capable as the 410's I've used over the years but the tradeoff was top end performance and weight. It was huge and heavy and needed heavy treble boost to cut through a mix. Since then I put the JBL in an old Ampeg/Peavey/???? cabinet I found on craigslist. It had significantly less volume than the Mesa and was not tuned well for the speaker to maximize its low end performance. But it was 50 lbs lighter than the Mesa, fit in my Accord's trunk, and was still loud as hell with 300 watts.
The next 18 I used was in a Thunderchild 118 built by MAS. My model is somewhat different than current production cabs but weighed 100 lbs less than its Mesa/JBL counterpart. This cabinet was so clean and clear that I found myself wanting a more colored cabinet for most of my musical ventures. I once again need a clean "big rig" so it'll be coming out of its early retirement soon. It is nothing like your typical perception of an 18" cab in terms of sound, weight, and size.
The final 18 was found in a B18N. It is an AlNiCo speaker with a wooly, warm character like most would expect from an 18. It is not really any louder than its B15N equivalent in any regard. It is just bigger, heavier, and has a slightly different (but equally awesome) character.
In summary, 18's are just another means to the same end: amplifying your instrument. They vary just as much as any other size speaker in terms of weight, performance, and voicing. You just need to find what works for you.
