Jim (Broadstone), according to the manual the 300SE is rated at about 10 watts, which actually is a bit more than is typical of many amps which utilize a single 300B output tube per channel. Cary also produced a 300SSE stereo amp, BTW, which used two 300B's in parallel per channel, rated at 20 watts.
Also, if the sensitivity of your 3A speakers is anything like that of the currently produced Reference 3A MM De Capo BE, a note of caution: Based on these measurements the 92 db sensitivity rating of that speaker is arguably optimistic by as much as 6 db (which is a factor of 4 in power terms).
Finally, keep in mind that the most important factor affecting how much power may be required will often and perhaps usually be the dynamic range of the music that is listened to (i.e., the DIFFERENCE in volume between the loudest notes and the softest notes, with well recorded minimally compressed classical symphonic music often being the most challenging genre in that regard).
Best regards,
-- Al
Also, if the sensitivity of your 3A speakers is anything like that of the currently produced Reference 3A MM De Capo BE, a note of caution: Based on these measurements the 92 db sensitivity rating of that speaker is arguably optimistic by as much as 6 db (which is a factor of 4 in power terms).
Finally, keep in mind that the most important factor affecting how much power may be required will often and perhaps usually be the dynamic range of the music that is listened to (i.e., the DIFFERENCE in volume between the loudest notes and the softest notes, with well recorded minimally compressed classical symphonic music often being the most challenging genre in that regard).
Best regards,
-- Al