I am a clarinetist, soprano and bass, and I have recently acquired an old Conn M16 Director Tenor Sax. I took it to a very experienced technician for adjustment and a few new pads. He test-played it O.K. However, I am quite unable to play the D,C and B of the low register, it plays an octave higher. The second register plays these notes well in tune, so the technician did a good job on the pads. I am using a new Fobes Debut mouthpiece and I have tried various reed strengths from 2 1/2 up to 4. I have also adjusted reeds with my reed knife and sandpaper, ( I am experienced at this with my clarinet reeds). Nothing has helped so far.
I would value advice from experienced sax players. Many thanks.

Well i have never run into a problem like that before. There are many things that could be occurring. #1 is making sure your embouchure is right. Going from clarinet to sax is not an easy jump to make. they may seem similar but there are some differences. #2 did your tech test the horn with a leak light. a lot of times there is a bad seal on one of the right hand stack keys. I know from experience that going to a clarinet from sax is not easy but i have adjusted. Hope something i mentioned helped.

Throw a champagne cork in the bell and see if anything changes (it probably won't).
If not, take it back to the technician and see if he can play the notes and/or put a test light in it.
If he can play the notes then it's you.

I know I have done repairs and play tested the instrument, then made some more small adjustments and stupidly failed to play test completely again - - - only to see the instrument back in my hands. It's a small silly mistake. But if he just re-padded it, it's his responsibility to be sure it works (or tell what doesn't and why it doesn't). Perhaps a pad shifted afterwards because the glue didn't set properly.

Make sure you are dropping your jaw for these low notes as you must on the bass clarinet. Also, check the mouthpiece angle. On clarinet, the mouthpiece comes out at about a 45 degree angle. Saxophone mouthpieces come directly out of the mouth at almost a 90 degree angle. If the mouthpiece is at too low of an angle, you will have trouble with low notes, which is currently the problem.

Get an experienced sax player to try the horn - that's the easy way to check whether it's you or the instrument.

It is most probably the reeds fault. I have about 20 reasonably good reeds and using the digital tuner I adjust the mouthpiece to get a perfect B and then move to the other notes and check to see if they register true on the digital tuner. Some reeds give perfect notes all through the range while other reeds start on a perfect note but are unable to make the other notes perfectly. It's due to the reeds not being conditioned properly and so they are unable to vibrate or flex fully and they make higher notes instead of the note they're supposed to make.

The trick is to condition the reed properly before you try to play all the notes with it. The best way I've found to do this is blow through the sax without pressing any keys for a minute or two and then press the first key and blow through the sax for another minute or so. Then slowly move to the other keys one at a time giving the reed time to adjust to each note for a minute or two. Then when you finally get to the lower notes you'll find the reed plays them easily.

If you just put a reed in and expect it to reach all the notes straight away then only 1 out of 5 or ten reeds will be able to do that from what I've experienced. Most reeds need to be warmed up or conditioned in the way I just told you.

I found that out by accident while playing my clarinet. If you just start blowing without pressing any keys and then gradually press the other notes one by one, giving the reed time to adjust fully to each new note, then the reed will perform a hundred times better and you won't have to blow so hard either.

I've found this method works on tenor sax, alto sax and clarinet. I haven't read anyone else using this method, I discovered it by accident.

So your sax is not at fault it is simply that some reeds cannot reach all the notes without being warmed up or conditioned properly first.