Here's a practical example. Toshiba's 2SA1302 power transistor, for example, with 100 mA of current will go up to about 17 MHz, with 150 mA up to about 21 MHz, with 200 mA up to about 25 MHz and with 300 mA over 30 MHz (as per manufacturer's data sheets). While a transistor's speed is not an exclusive function of its response, the response is the most important factor, of that there's no doubt. Obviously, using more bias will improve its response and shorten its reaction speed.
As a general rule, the factory set point will usually be the lowest acceptable point. In most cases, increasing the quiescent current will yield beneficial results, such as improved speed, better coherence, more to much more spatial information and usually a better, clearer bass and less distortion, both static (THD, IM) and especially dynamic (TIM, TID, SID, etc). However, it will also cause the output stage to produce more heat, so one must be careful not to overdo it. Increased heat is good, very desirable up to a point, since it will allow the amp to work under more even conditions thermally, but I repeat, up to a point only. Also, you will reach a point above which you can go on increasing the quiescent current, but will receive very little, if any, sonic benefit; this simply means that you have reached and gone over the truly optimal point.
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