sub woofer efficiency

If you have a 98 db efficient speaker what efficiency subwoofer do you need to
keep up?
Look for a sub with a good bit of power. You shouldn't have much trouble matching sub to loudspeaker. The problem is finding subs that sound good for music if this maters to you as it does me.
I agree with Johnk that finding a sub that will match level-wise isn't going to be difficult. Subs typically have built-in amplifiers that have lot of gain, so the actual electrical efficiency of the woofer isn't an issue because they're designed to level-match with a wide variety of speaker systems. If in doubt, tell the sub manufacturer what you plan to do and ask for his opinion.
its depends on speakers if they are fragile paper cones 88db which can handle 20wats i would dsay no diference. but if they are able to take some power ,practicaly most subs will fail/burn/smoke/crackle. in the second case i would look for sub with no less than 115db/30hz.
also keep in mind that dicortions in lower range is less noticebale but at hefty levels(which are needed for 83-86db subs) most subs have 10%-15% discortions which is realy too much.
these 2 efficiency ratings have nothing to do with each other. audiokinesis is correct unless you buy a sub without an amp, which is not easy in todays market. any sub with enuf guts will work, finding a 'musical' sub is just jargon for finding one that is well designed and offers flat response and low distortion levels. most actual deep bass notes occur in the 30-40 hz region anyway so more distortion below this is not gonna be disastrous at most loudness levels anyway. lots of nitpicking with subs but well made subs between 500 to 1000$ will generally work fine if you take the time to place them in sweet spot and set controls right. acoustic room treatment becomes very important tho when you start pumping in really low sound waves....