If memory serves, there are a total of 4 output transistors in this amp?
If you can get the MJH11019 & 20 that should be good.

Yorkville switched to MJH11017 & MJH11018 in some of these models, that would be the best sub.
However, you should contact them and ask.  In the higher power amps like the 150, it may not work.  The spec of the original BDV66 was changed and subs for the later type can be trouble.  They actually had a mod to add an extra output device when repairing certain models that used the BDV66 & 67.  Don't recall if it was the 150B, 250B, or both?
Best to contact them, or gamble and keep your 66's and use the MJH11018 for the 67's.

  Some of those Fenders use the heatsink "bar" that mounts to the chassis.  From what I recall, it can be mounted upside down and if so, doesn't mate correctly to the chassis.  Or there can be dried up heatsink compound that interferes with proper heat transfer.

  So sorry to hear this.
Condolences to family and friends, he will be greatly missed.

  To speculate a bit about the etymology of "rail":
in electric railway (and subway) systems, the power is taken from the "third rail".  This goes back over 100 years.

  Is this schematic close?
http://s292.photobucket.com/user/ledzeppy/media/Tone_Bender_MKI.jpg.html

  For the SM2177 you can use a PNP complement to the Tip47.
Or some use MJE15033 in the HH with no problems (and you can use MJE15032 for the Tip47).

Edit:  sorry Enzo, we were thinking the same thing at the same time. 

  Before throwing away those priceless old Fet's, I'm not completely convinced the 134 is bad.  Fet's can be difficult to measure, and will sometimes show shorts depending on how they are tested.

And further to the good points RG and Roly have presented, you can't make extra power out of thin air.
Those regulators need an input voltage maintained at 3V more than than their output, so circuit adjustments may be required. 
This is assuming the power transformer has the capability to provide the extra voltage/current.

  A simple thing to try is using a spare cord to connect the FX loop send to the return.  Does that make any difference?

Here's some more reverb tank info you may find interesting, includes some sample circuits:
https://www.amplifiedparts.com/sites/all/modules/custom/tech_corner/files/spring_reverb_tanks_explained_and_compared.pdf

Is it a Medalist 4/10 like this?
http://www.ebay.com/itm/VINTAGE-AD-SHEET-2159-1968-GIBSON-MEDALIST-4-10-AMPLIFIER-/400606908345?rmvSB=true
If not, list the tubes that are in it.

  I think he left out the "62" in that link.
Here's a direct link to the pdf:
http://www.bonisaudio.com/code/Bonis_Audio_Doc.zip

  If Dr.Gonz' schematic is the right version, then E13 comes off one side of R21.

  So I guess that 550W from the amp must be peak or something?