If you try too hard to polish a piece of ****, you just gum up your tools and get a lot of nasty stuff under your fingernails. I can't think of any real reason to try and use a horizontal output transistor for audio, except perhaps in Class A as a direct drive for an electrostatic speaker, (and that's a pretty big stretch of the imagination), or just maybe as an intellectual exercise if I had nothing else to do. I just pulled out my old Sanyo data book and took a look at the horizontal deflection transistors thay offer. Gain is listed a >8 at 1A collector current. - pretty shabby. I actually have some old 1500V Motorola horizontal deflection transistors that have a gain of one (no typo) at their rated drain current. I keep them around as curiosities, and they will probably still be sitting in my parts cabinet when I die, if I don't throw them out first.