Bottomdweller...I just bought an Apeks 3-stage set which included the TX50, TX40(octo), and the DST 1st stage. I absolutely love them. Very easy breathing, comfortable and breath dry. There exhaust port rests on your chin taking alot of the load off your jaw, which increases comfort. The differences between the TX50 and the TX40 are that the TX50 has an inhalation adjustment knob that the TX40 doesn't, giving it more adjustability. But honestly I couldn't tell any difference in breathing between them at any depth up to 100', that's as deep as I've had them so far. But another advantage to the adjustment is that you can back the adjustment all the way out during storage and prolong the life of the low pressure seat.
About the differences between the Apeks and last years Zeagles....I asked this same question here a while back and you can read the thread
here. Here's what the dealer that I ended up buying the Apeks regs from had to say about the differences between them:
The differences in the OLD STYLE Zeagle regs and the NEW style Apeks regs are subtle, but substantial in my view. Changes made to the NEW Apeks regs (those sold after the start of 2000) are not available in the old style Zeagle regs (which were all supplied to Zeagle prior to october of 1999) differences include the shuttle valve spring, the shuttle valve, the counterbanance cylinder, and the high pressure spring in the first stages. In addition, the orings in the Apeks are made from a "new" material they
don't much talk about. We know they aren't butyal, silicone, or viton, but they don't talk much about what they actually are.
I never confirmed this with Apeks, but I believe the above to be the truth and I could tell the difference myself after diving both the Zeagle Tech 50D and the Apeks TX50. The first thing that I noticed was that the Apeks breathed easier IMHO. The second thing that I was relieved to discover was that the Apeks TX50 and TX40 both breathed dry, the Zeagles that I had dove did not when inverted the least little bit. Granted the Zeagle I used several times was rented and it wasn't new, far from it, and the Apeks was new, so that could have had alot to do with it.
BTW, Zeagle no longer markets the Apeks line, they are now making their own regulators. I here through the grape vine that their new regs are top contenders and will recieve the highest breathing machine scores of any reg ever tested, but that's just heresay for now, we'll see. And I for one, would be very carefull about jumping on the bandwagon of a new reg from a company that has never actually made regs until now, but that's just me. Just so you know, I'm not anti-Zeagle, I own one of and love their BCs.
