[Arran]

7A4's can be used in place of the XXL's.

 A 7A4 is the same tube as an XXL, except that was how Sylvania and others chose to label them. The XXL, XXD, XXB, XXFM, is just a load of B.S from Philco's marketing department, every one of those has a normal RMA standard number equivalent. Philco was on this kick back in the 1940s of using triode tubes for both mixers and oscillators, in the case of the XXD/14AF7 and XXB/3C6 they had the two triodes under one glass, the XXL/7A4 was single triode under one glass so they used at least two in every radio that used them. Why did they make two a dual section whilst the third a single section? Tube stuffing the chassis I guess, why make a six tube set when you can make it seven and sell it to people as a higher end product? This is much like the use of 6H6 tubes along with a 6J5 rather then a 6Q7, it's a racket.
By the way, because they used a triode as a mixer or first detector, they lost some front end gain, so to make up for it they usually added an extra IF amplifier stage, which was cheaper to add then a tuned RF amplifier stage although they could have used an untuned one. Then again they could have used a seperate triode and used a heptode as an oscilator and mixer combo, and avoided having to add that extra IF amp stage, or left it in and ended up with a better set, but I suspect that Philco did a lot of these things either for marketing purposes or for patent royalty avoidance. One thing that I have noticed about engineers of all sorts is that sometimes the designs they come up with really have no underlying reason other then personal prerogative.
Regards
Arran