Sean is right and Sean is wrong. A watt is indeed a watt. Also, many years ago the Federal Trade Commission regulated the way manufacturers rated the power of their consumer audio equipment, so generally, one at least gets the "100 watts into 8 ohms from 20 Hz to 20KHz at no more than 0.1% harmonic distortion" kind of rating. (Previously manufacturers rated amplifier power any way they wanted and usually they didn't tell you enough.)
However, Sean is correct about tubed and solidstate amplifiers behaving much differently driving different loads. Because tubed amps have output transformers, they have relatively high output impedance, while SS amps generally have very low output impedance. Within its construction limits, a SS amplifier will continue to increase its current output and maintain its voltage output (hence increasing its power) as the load impedance decreases. This means that about-any SS amp that's driving 1 watt into 8 ohms will drive 2 watts in 4 ohms, 4 watts into 2, 8 watts into 1, 16 watts into 1/2 ohm, etc. Of course, the higher amount of power (and current) we start this exercise with, the quicker the amp will reach its limitation on current flow. 'High current' amps will indeed increase power as the load impedance decreases, and the good ones will double rated power at least once. For instance, an inexpensive amp may have only one power rating, usually into 8 ohms. If it doesn't have another rating of higher power into 4 ohms, its not a high-current amp IMO. Some will have more power at 4 ohms but not double the 8-ohm rating. The better amp will indeed double its power into 4 ohms, and the really high-current designs will double again into 2 ohms. For example, my humble little (but HOT-running) Lazarus hybrid amp (with SS output stage), is rated at 50 watts per channel into 8, 100pc into 4, and 200pc into 2 ohms. It costs lots more money to design and build an amplifier that will do this.
Tubed amps can't do that. A tubed amp will increase power slightly as load impedance decreases but not by much.
But still, a tubed amp will sound somewhat better when driven to deliver power beyond its rating (ie, at clipping) than a SS amp will; many people say that a tubed amp clips more gracefully. Thats why many of us say that generally one can get away with a tubed amp of half the rated power compared with a SS amp. However, the lower the impedance of the speaker (and hence the more current the amp has to dump into it to play music as loudly as the listener desires), the less likely would a tubed amp be able to cope gracefully and sound good. Of course, each amp of either type will behave differently and sound different under various real speaker loads.
Also, Gs5556 is absolutely correct, and lots of us pay little attention to the relative efficiencies (sensitivities) of speakers. Every 3dB of higher or lower sensitivity requires double or half the power to drive to equal loudness. My 96dB-rated Kindel PLS-As require ONE-QUARTER the amplifier power than a 90dB-rated speaker to drive to equal loudness. Thats why my little 50wpc Lazarus amp will drive them to deafening levels. I dont wish to belabor this point, but a not-untypical 87dB-rated speaker requires FOUR-HUNDRED watts to generate the same loudness as my 96dB/50-watt combination.
So Blindjim, start your evaluation process by picking the speakers first, understanding their sensitivity ratings, and then picking some amplifiers to listen to. Unless the speakers impedance dips below, say, 4 ohms over a significant part of the frequency range, a tubed amp will probably do very well.
And my answer your original question is 'enough to avoid amplifier clipping about 99% of the time'. Yes, that's not very useful, but YOU have to answer it, not us. :-)
However, Sean is correct about tubed and solidstate amplifiers behaving much differently driving different loads. Because tubed amps have output transformers, they have relatively high output impedance, while SS amps generally have very low output impedance. Within its construction limits, a SS amplifier will continue to increase its current output and maintain its voltage output (hence increasing its power) as the load impedance decreases. This means that about-any SS amp that's driving 1 watt into 8 ohms will drive 2 watts in 4 ohms, 4 watts into 2, 8 watts into 1, 16 watts into 1/2 ohm, etc. Of course, the higher amount of power (and current) we start this exercise with, the quicker the amp will reach its limitation on current flow. 'High current' amps will indeed increase power as the load impedance decreases, and the good ones will double rated power at least once. For instance, an inexpensive amp may have only one power rating, usually into 8 ohms. If it doesn't have another rating of higher power into 4 ohms, its not a high-current amp IMO. Some will have more power at 4 ohms but not double the 8-ohm rating. The better amp will indeed double its power into 4 ohms, and the really high-current designs will double again into 2 ohms. For example, my humble little (but HOT-running) Lazarus hybrid amp (with SS output stage), is rated at 50 watts per channel into 8, 100pc into 4, and 200pc into 2 ohms. It costs lots more money to design and build an amplifier that will do this.
Tubed amps can't do that. A tubed amp will increase power slightly as load impedance decreases but not by much.
But still, a tubed amp will sound somewhat better when driven to deliver power beyond its rating (ie, at clipping) than a SS amp will; many people say that a tubed amp clips more gracefully. Thats why many of us say that generally one can get away with a tubed amp of half the rated power compared with a SS amp. However, the lower the impedance of the speaker (and hence the more current the amp has to dump into it to play music as loudly as the listener desires), the less likely would a tubed amp be able to cope gracefully and sound good. Of course, each amp of either type will behave differently and sound different under various real speaker loads.
Also, Gs5556 is absolutely correct, and lots of us pay little attention to the relative efficiencies (sensitivities) of speakers. Every 3dB of higher or lower sensitivity requires double or half the power to drive to equal loudness. My 96dB-rated Kindel PLS-As require ONE-QUARTER the amplifier power than a 90dB-rated speaker to drive to equal loudness. Thats why my little 50wpc Lazarus amp will drive them to deafening levels. I dont wish to belabor this point, but a not-untypical 87dB-rated speaker requires FOUR-HUNDRED watts to generate the same loudness as my 96dB/50-watt combination.
So Blindjim, start your evaluation process by picking the speakers first, understanding their sensitivity ratings, and then picking some amplifiers to listen to. Unless the speakers impedance dips below, say, 4 ohms over a significant part of the frequency range, a tubed amp will probably do very well.
And my answer your original question is 'enough to avoid amplifier clipping about 99% of the time'. Yes, that's not very useful, but YOU have to answer it, not us. :-)