I guess it depends if your taste is live rock or chamber music so to answer your question we need a lot more information.
Factors like X-max, efficiency, Fs, Qts and baffle Fc are all important.
30 watts is quite low for an open baffle bass driver, especially considering the bass response will roll off 6dB for every octave below baffle Fc.
I use two 15" / 800W woofers per channel and for me it lacks headroom, even when listening at moderate levels. (maybe 100dB SPL)