While it could possibly be argued that using an amp to compensate for a poor aerial system is in some sense inelegant, it is not in fact an unreasonable strategy even for PSK or the like. Obviously the amp must be clean, but that really should be the objective anyway.
For PSK mode, I must disagree with this. Using an amp IS an unreasonable strategy for PSK for a variety of reasons. The most important being that IMD in PSK mode isn't some imaginary thing that may happen or not. Even when unmodulated, an idling PSK signal with less than excellent IMD has sidebands that will interfere with adjacent stations. Often, we hear someone use the term "clean" as you have here applied it to amplifiers. It might surprise many to learn that some common and expensive amplifiers have IMD ratings that aren't exactly excellent. For example, the Acom 2000 (IMD rated at 31 dB), Icom PW1 (IMD rated at 29 dB). Would you say those are "clean" amplifiers in terms of IMD? Using an amplifier for PSK mode is just a bad idea, even with a poor antenna.
73, Floyd - K8AC
I really do not think that a amp is going to cause a IMD problem unless it is improperly driven or loaded. (BTW the IMD figures you gave for amp was at rated output, not reduced output) PSK is not that sensitive. As to whether amp is needed is debatable but there are times when even a good antenna is not enough for poor conditions. Personally I do not do much PSK myself. I prefer other digital modes.