VVCV, There are several factors that determine the speaker sensitivity needed to use 45 tube amps. Many have already been
stated. Room size, music density, max SPLs for dynamic peaks, average listening volume, etc.Keep in mind that many manufacturer's speaker sensitivity ratings are for one central frequency. As ddriveman suggested the lack of headroom for dynamic peaks is often the limiting factor. Speakers without crossovers can help. Passive crossover can sometimes be a more difficult load and can consume power. A flat impedance curve and minimal phase anomalies help.
I use mostly low power SET amps to demo our speakers which have a sensitivity of 105 dB. We have two pair of 45 mono amps, a pair of 2A3 mono amps, a pair of 300B mono amps and a few other amps of various types. Our showroom is 21 feet by 31 feet with a ceiling that starts at 9 feet high and goes to about 14 high feet at the rear. Our speakers have separate powered woofers so the SET amps don't have low bass duty. The main towers go down to about 100 Hz but we typically set the active crossovers to 120 Hz. The amps are directly coupled (no passive crossovers) to the drivers. The 45 tube amps, to me, sound the best and have no trouble driving the towers to high volumes with no compression. In our setup the 2A3 amps are a close second. Our speakers are a line source so the dB loss per distance is less than a point source speaker which helps.
Our speakers are unique in many ways so maybe not the best example but typically for a medium size room, listening to reasonable/enjoyable volumes a minimum sensitivity of 100 dB should work. With so many variables you should audition the potential combos.
I use mostly low power SET amps to demo our speakers which have a sensitivity of 105 dB. We have two pair of 45 mono amps, a pair of 2A3 mono amps, a pair of 300B mono amps and a few other amps of various types. Our showroom is 21 feet by 31 feet with a ceiling that starts at 9 feet high and goes to about 14 high feet at the rear. Our speakers have separate powered woofers so the SET amps don't have low bass duty. The main towers go down to about 100 Hz but we typically set the active crossovers to 120 Hz. The amps are directly coupled (no passive crossovers) to the drivers. The 45 tube amps, to me, sound the best and have no trouble driving the towers to high volumes with no compression. In our setup the 2A3 amps are a close second. Our speakers are a line source so the dB loss per distance is less than a point source speaker which helps.
Our speakers are unique in many ways so maybe not the best example but typically for a medium size room, listening to reasonable/enjoyable volumes a minimum sensitivity of 100 dB should work. With so many variables you should audition the potential combos.