There is no hard and fast "right answer" but there are common guidelines. As swampwalker suggested, space/volume will have an effect. Speaker impedance (and how it varies by frequency) could also play a very strong role. Second, not all amps of X watts are created equally (power supplies may affect amp response to speaker load).
That said, I find the article to which Kehut refers above to be at best oversimplified and misleading, because I have think most speakers with 2x that sensitivity (87db/6ohms) would present trouble with the amp being reviewed with music which had real "needs" (Holst Planets' Saturn, Reger organ music, and the list could obviously go on). 845s are supposed to have more oomph at the low end than other SETs but my personal experience with SETS and relatively efficient speakers (96db on the bass, far more on the mids/tweets) is that one needs to test even higher-efficiency woofers with more power than recommended to see if one is getting the best out of the speaker. My experience with 96db woofers and 13 watts (despite a power supply I rate quite highly) suggested that to get real clarity out of the bass (and bass that goes deep with clarity makes the the rest of the spectrum stand out that much more - positively shiver-inducing in my book) I needed probably double or quadruple that at times. 100% of the time while listening to Mozart chamber music I would be fine. And even most of the time on orchestral music I would be fine, but when push comes to shove comes to tympanies pounding, cymbals crashing, and the organ in the background grinding out the low notes, I needed more oomph.
Of course, that and a dollar will get you a dollar cup of coffee.
