Sansui au 2900 for dedicated 2 channel vinyl?

The AU 2900 will work fine for a small room system. It is a very good, entry-level vintage amp. If it were me I would look for an x17 (117 through 917) model; they will be in the same price ballpark but are substantial units.

At low listening levels you can use almost any speaker but I would suggest a pair of classic speakers like Dynaco A25s or Sansui SP-200s if they are not to big. Another good option would be a pair of Spica TC-50s.

Classic Sansui amps are sound great and, if avoid the products made in the 80s, are built like tanks. Have fun!

Congratulations on your AU 2900 purchase. You mentioned that this system is going into your office. How big is the room, where are the speakers going to be placed, what type of music do you listen to and how loud?

If you are going to be in a smallish room, playing music at moderate levels I wouldn't be too terribly concerned with speaker efficiency.

Klipsch Heritage designs are great and economical, but they tend to be rather large and may not work well in a small room; they are definately not bookshelf units. KG4s would be a good match for your amp, if you have the space.

If it were me, for a room of that size and given your musical choices I would probably go for an "old school" speaker:
1- I still think that the A25s will work great, but so will the SP-200s.
2- Another good, low cost option is the Boston Acoustics A60, A80 or A100. They all sound similar except for the increased bass response as you go up the line.
3- The Snell K/II, J/II and E/II are great, undervalued speakers. They sound good, are bulletproof, easy to place and cheap.
4- My opinion is that your room is probably too small for vintage Klipsch speakers. There are those who would disagree, but they do pump out a lot of bass and can overwhelm a smaller room.

Good luck.

1- Your amp is a stereo unit, not quad. You are seeing 2x 2 channels (A & B) - that's very common on 70s/80s consumer gear.

2- I believe that your amp is rated at 25W per channel. I could be wrong, but that is what is listed on one of my reference documents.

3- I've already said all that I need to say re: the Dynaco speakers. Find a pair and give them a try or go to something else. This is vintage gear - there are no guarantees. If you want guaranteed compatibility you should probably be looking at new gear, or at least gear that is still supported by the manufacturer.