Playback volume changes in solo mode

I'm using HS6 on a laptop running Vista (I tried 7 but couldn't get used to the interface and didn't like the loss of CAL and set-playing capability).  I work with all-midi (no audio) sequences.
Whether I insert the TTS-1 softsynth or just send output straight to the Microsoft wavetable synth, when I solo a midi track, it sounds much louder than the same track sounds when others are playing.
Also, if I have a part - piano, for example - playing at constant volume, it gets significantly louder if other tracks - e.g. bass and drums - rest for a few bars, and then the piano volume drops back to normal when the other instruments come back in. 
The problem is especially noticeable and annoying if I'm starting a song by bringing in one instrument at a time.  It's almost as if the program is trying to keep its total output at the same level, no matter how many instruments are playing.
Has anyone else experienced - or better yet, solved - this problem?

Limiter in master bus? Not that I'm aware of, but I'll check it out. Thanks for the reply.

Good deal. Glad you got it figured out.
And thanks for posting your solution. That's the sort of thing thats sure to get somebody again.