V. Interesing observations.
I'm not a bedroom level kind of player, and I use the boost for actual solo boosting all the time, and I've never noticed a tonal difference. I think that what you may be hearing is the oversaturation from the boost being loaded by very high resistance b/c you're playing @ such low SPLs.
One thing to note: When you use the solo boost on a Splawn, it switches to OD2. So, if you're playing in OD1, you will notice a tonal difference. If you're already playing in OD2, there is no change.