The 6AK5 was designed for low noise amplification of low-level VHF/UHF signals.
So?
Audio distortion was not a consideration.
It wasn't a consideration for the design of the
vast, vast, vast majority of VT types out there. Even those types that have the rep for sonic excellence, such as the 6SN7, were designed for other purposes. Just
essssssss-loads of 6SN7s were used in early radar sets and the first electronic computers. 6SN7s also showed up as vertical deflection oscillators/amps in TV sets, eventually resulting in the 6SN7GTB, with hardened control grids to stand up to positive Vgk operation, and with the plate dissipation raised from 3.5W to 5W as TV screens grew larger.
The 6BQ7 was another type designed for use as a VHF small signal amp (good to 300MHz) with absolutely
zero audio pretense, as this type of operation is never mentioned in the spec sheet for the type. Well, guess what? They sound great in the right applications.
That old reliable audio work horse, the 6AU6, was originally intended for use as an RF amp.
How any one specific type sounds has more to do with loadlines and application, rather than for what it was originally designed. Some RF types, large signal/small signal, can sound great; some will sound like [IMGDEAD]http://img225.imageshack.us/img225/6457/pukeface3kz.gif[/IMGDEAD] no matter what you do.
Other specialized types that have no audio pretensions (e.g. horizontal deflection types -- 6BQ6GTA sound just as good as the famous 6V6,
and put out twice the power: no audio operating points mentioned in the spec sheet) can work just fine. Finding these audio "sleepers" is well worth it since they aren't audiophool expensive (yet).
Worth a try.