From a post I made last November:
In October I did a check with an 8" off-axis mask on the 20" Dob at a dark site (21.3 MPSAS at the time.) I first located the four largest moons with full aperture, then put on the 8" mask. Seeing was about 5/10 Pickering for 8" aperture at the time.
At 278x Titania and Oberon were readily visible. At 357x I was detecting a brighter spot in the glare surrounding the planet. This was Ariel, and 500x provided sufficient image scale to separate this very faint but bloated spot from the glare. In addition I realized I could also detect Umbriel preceding the planet. Umbriel was visible about 50% of the time in averted vision and could be held at moments, so there was no ambiguity about it.
This was in mediocre (but decent for here) seeing that was quite variable that night. I would rate it more as passable than good for this type of observation. This wasn't one of the darker nights at this site either, actually this was toward the brighter end, although sky darkness is less of a factor for seeing moons like this (once the sky is darker than ~21 MPSAS) than the glare of the planet which becomes the major limitation...along with seeing which contributes mightily to the apparent glare.
I had no idea I could pick up Ariel and Umbriel with 8" (unobstructed). However, I did so after seeing them first with the full 20" aperture, so I knew where to look, particularly for Umbriel. I went well past magnification I typically would use for an 8" for planetary or these seeing conditions--in this case to provide image scale for separation alone. My SCT with ~34% obstruction was typically limited to about ~300x for planetary detail in excellent seeing.
There is little doubt to me that Titania and Oberon would have been visible with a 6" mask on the 17 year old reflector. Unfortunately, I had left my 6" mask at home that night.
Actual good or excellent seeing would make such observations more straightforward.