First, tell your guitar players to TFD (turn down please).
Second, you need more speakers. Another 410 on top of yours (ideally the same model) will give you more volume and cleaner headroom. I assume you have the Hydrive, and if you do that's an 8ohm load, meaning you're only getting about half the watts out of your GK. Watts don't equate to volume, but they equate to cleaner headroom, which is to say more in the way of clear notes. You're cranking your GK to fight for volume out of your four speakers, and when you crank a GK you get grind and a bit of distortion. That can be awesome sounding, but in your situation it sounds like you need more headroom and volume. A second 8ohm 410 will get you a lot more volume, and because you will be running a 4ohm load with the two cabs together, you'll be using the full wattage from the GK and getting that extra headroom.
A 15 on top might sound okay, but fighting guitars I'd put another 410 on there. Low end response has more to do with cab design rather than speaker size, as the wisdom goes.
Eat all your veggies. And don't bang the dillies.