You check the difference between the two mid pots......

The one that is 'working' could be a 250K Linear
The one 'doesn't' could be a 250K Logaritmic

Maybe, thats the difference - check it! :-)

Or you could change the 1M master vol pot to a pot with push/pull swich.

Put the bright cap on the switch and when you use the Dlator, you pull the master pot to engage the bright cap and restore some of the lost top end.

This way your amp will look stock.

I play mainly Strats and Teles and in my exprience you can get very nice clean tones with both low plate and high plate skyliners (like the OTS) with the lowplate skyliners (100k/1.5k on V1 and V2) having a tad better clean tones.

Before I got into Dumbles, I thought that my old Bassman head, was the epitomic of clean and blues  tones. Now I think its got too much unusable low end and top end as well as a lack of middle tones, which is why the Tubescreamer (a mid boost monster) is so popular with Fenders.

I my opinion 80s and 90s Dumble are designed like Fender (blackface and silverface) should have been in the first place.

I'm a bit red/green colour blind myself and when in doubt, I ask somebody else which colour is which. 

I really like the
"
...with the Dumble, you can play at any volume and get all those overtones. With other amps, you have to open them up really loud to get the richness. Bring them down, and it’s like a big balloon that’s not inflated. Dumble? No, he’s got balloons at any size you want."

and

"...With its 12" Altec, the Boogie is a soprano. It gives me throat tones, nasal tones, and head tones, but not chest or belly tones. But now because I use four 12s with the Dumble head, I get the Pavarotti and Placido Domingo belly tones."

Only a true artist can come up with pictures like that. It reminds me of the 1985 Alexander Dumble interview in Guitar Player, where he compares tone fullness with a football field full of marshmellows.

Santana and Dumble are two comparable artists, although Santana is probably richer. 

I have a new favorite, which can easily be build into your amp:

http://uk-electronic.de/onlineshop/product_info.php/products_id/1478?osCsid=11685b8d41e8b5d3521dc446d159699a

It sounds great and price is ok.

Diferent 'brand' same Belton reverb unit:

What is a flexiplex OD 

So in essence your preamp is a standard HRM?, but the poweramp is a BM

The jacks are wired in parallel, so it doesn't matter if you use the main or extra - even with only one cabinet.

And, yes, if you connect two 8 ohm cabinets to the amp, you should switch the impedance to 4 ohms.

It will probably be on par, when you've put as much time into it as you have on your guitar chops.

Ok, cool

Are you sure about the skyliner. Usually Bluesmaster has its own tonestack more or less derived from a Fender Bassman/Marshall ditto.

Hi Welcome to the forum,
You need the Special (OTS) model, thats the route to Santana Sound.
Regards

PS: Great user name, you've selected, maybe you can help select a goo cognac someday 

Great sound, great playing
What kind of Dumble amp type is it?? Bluesmaster?

You need a 'mixing' resistor in order to keep the direct and effected signal seperated, but try it without and see if you like it like that.

You can also experiment with the resistor values, if you want more or less output