[regal]

I am considering building a 45SET headphone amp and with a little extra work it could have a switch over to drive speakers around 2W's.
 
I know these types of amps are popular but I have never really seen speakers that have the efficiency to work with such a low power.
 
I have a pair of "bookshelf" Fostex 127E's,  would these be suitable if I added an active sub?
 
 
Also it is a mystery to me when I read a review of a 45 SET tube amp and the reviewer talks about his 110dB speakers,  I have never seen plans for a speaker with such efficiency not have I ever seen a commercial speaker like this.   Its either a closely guarded secret or exaggeration?

 

[n3rdling]

I know horns are very efficient, and those are typically the speakers paired with SET amps.  For me personally, 2W would probably be enough to reach my listening level with most speakers; it really depends on how loud you listen.

 

[Jon L]

I suggest that you design your amp optimized around something along the lines of EML 45 solid-plate tube.  You really won't want to use any other 45 tube after you hear the EML.  Even with NOS regular 45 tubes, it will be loud enough in a reasonable room with your Fostex, which doesn't have much bass anyway.  More headroom if you cross over to active subs a little higher than the usual 70-80Hz.  
 
For example, I used to power 95 dB/7Ohm speakers from 140 Hz and above with 45 SET, with bass powered separately, and this was loud enough for me in moderate rooms.  Do remember that 2 watts is 2 watts, and if you want to crank Mahler at ear-splitting levels, 110 dB horns are more appropriate..