Hi Jon,
My story...6 months ago I went down a similar path with my Lunt 60mm. No detail and no proms no matter what I did.
I replaced the orings and still nothing. My blue/green glass in the blocking filter was clean and not clouded.
I have a 40mm Coronado Solar Max and it delivers stunning views, so, like you I became concerned. I tried another blocking filter in the Lunt and wow ...disc detail and proms. After speaking with Faye at Lunt it was decided to return my BF and it turned out there was indeed an issue with the blocking filter. As it was outside warranty it was a usd $180 repair. The view is now stunning. It was not a repair that could be done outside of Lunt ..was to do with the red looking glass of the blocking filter. I am not the only one here who has returned his blocking filter recently to Lunt for the very same issue as mine had.
FYI the cost to send back to Lunt by Australia Post was aud $20 and took 3 weeks to get there.... the return through DHL was another $ story but it came back within 7 days. Though another Lunt purchase sent by USPS cost usd $20 and arrived here in 28days.
As Martin points out it may well be the orings are worn or a clouded blue green filter as these are maintenance items. The orings can be bought from Lunt at usd $15 and the blue green glass for usd $8 or it seems it can be cleaned using CLR, but do some reading on IIS on this.
Assuming that your blue green glass in the blocking filter is not clouded, that you are getting focus (sharp edge of Sun), that you have undone the pressure tuner and screwed it back on and checked the Sun at various points of travel (mine screws down to leave about 10mm of the thread still showing to get good tuning) then I would see if there is anyone in Sydney who has a Lunt scope and would be kind enough to let you test their blocking filter in your scope to rule out any BF issues.
Also as you were thinking of getting a double stack..I see on the Lunt site they have a couple of refurbished 50mm double stack modules for your 60mm scope (same as mine) at usd $795 instead of new at $999.
PS, it happens I do have a set of spare new orings and am willing to send to you if you want to rule this out first. If they work happy days, you can send me a set from Lunt later (I would suggest getting 2 sets for future proofing!).
https://luntsolarsystems.com/replaci...tuner-o-rings/
https://luntsolarsystems.com/replaci...-glass-filter/
PM me with your mobile and I can go into more detail if you like.
Peter