Just as a word of advice, putting big-ass monitors in a room without treatment (even with treatment, the low end is the hardest to get under control) is going to cause more problems than you gain by extending your frequency response with bigger monitors - it's of course nice that the bass bumps, but if you're planning to mix in there, you should aim for accuracy instead - which might mean that, depending on the size of your room, smaller monitors may actually be better than bigger. Also, you have to spend a lot of money to get a pair of monitors that'll accurately reproduce 30Hz (without a sub).
Good advice. If you do have a small room and/or not want to spend energy in controlling bass reflection and build up, and you go for accuracy, this'll mean you won't be able to mix your low end properly. There will be a part where the low frequencies won't be hearable or not reliable.
30hz is indeed very low.
Don't always trust the listing of the low frequencies of the manufacturer either. On monitors like the focal alpha 80, jbl lsr 308, mackie mr8, yamaha hs8, you get about 35 to 40 hz. These monitors will lie around 200 to 300 bucks a piece.
Around 600 bucks speakers start to list frequencies as low as 25 hz. The new V 8 from krk, mackie hr8 (9 inch) or the others I mentioned in my previous post.
Mixing in an untreated room, lacking skills and theoretical knowledge, hardware, some software, is always tricky. I'd focus on getting one of those listed speakers where you have the money for and go from there. If you have a small room, controlling the low end will be a challenge. You'd solve that by going for a smaller woofer and sacrificing a part of the low low end. If you have a good room and willing to put some effort in treatment and setup, I'd definitely go with a 8 inch or bigger.
If you're in doubt, get a shop with a proper return policy so you can test it out in your room and the way you want it to be setup. If it doesn't work, you send it back and get a different pair.
Look up on some theory about bass build up and reflections of sound so you can recognize problems better. A lot of it can be solved by bass traps and such, but that needs energy, attention and money to setup. There's a topic somewhere on the forum on how to build it yourself. If you check youtube and google you'll be able to find information about this. On youtube, I like studio rescue a lot