[astro744]

If you need inward travel then an extension tube will only make it worse for then you will need to add the length of the extension to the amount if inward travel that you need.

I just tested with my ED80 (not original focuser as I bought used and previous buyer replaced the focuser for better astrophotography). In any case I was using a Bintel 2x Quartz Star diagonal and Bintel 2x Barlow early non-ED model. Without Barlow I have 33mm inward travel remaining. With Barlow I have 11mm inward travel remaining. I tried with the following eyepieces:; 30mm 1rpd, 40mm Meade 5000 Plossl, (both 2") and also with 25mm 1.25" Bintel Plossl. The 2"-1.25" adapter I used was the one that came with the Barlow, recessed type with zero length added. I got the same result to within 1mm.

I put the diagonal into the telescope first and then the Barlow into the diagonal. This gave me the above result. I then tried with Barlow into telescope first and then diagonal into the Barlow. I could not get focus at all, not even close at both ends of the travel.

I just packed it away and as I'm writing this I thought I should try with a 1.25" diagonal and 1.25" Barlow as well as a 2x, 2" Powermate. I'll try this later and report back.

Additional content:
I checked with ED80 with 2" diagonal with 2"-1.25" adapter and 25mm Plossl and then the same but with Tele Vue 1.25", 2x Barlow insterted into the 2" diagonal with 2"-1.25" adapter and remaining inward travel was 33mm. The Tele Vue 1.25" 2x Barlow is parfocal and adds zero length.

Your best bet is to use a 1.25" Barlow especially if only using 1.25" eyepieces. The 2" Barlow is taking up too much focuser travel that your ED72 does not have to spare.