The two models (66 and 99) use different types of tubes, so they will sound slightly different. So it will be partially subjective which you like better.
The Magnum versions comes with better stock tubes than the regular version. If you upgrade the tubes in a regular version you will be moving closer to the Magnum model.
Just judging by tubes alone, I like the sound of 6SN7 tubes. These are found in the 99 (four of them). So all other things being equal, it is likely I would prefer the 99.
The Magnum versions comes with better stock tubes than the regular version. If you upgrade the tubes in a regular version you will be moving closer to the Magnum model.
Just judging by tubes alone, I like the sound of 6SN7 tubes. These are found in the 99 (four of them). So all other things being equal, it is likely I would prefer the 99.