If you're sending too much signal into the front of an already (slightly) overdriven amp, many (not all!) amps will:
-just distort more
-flub out in the low-end
-become oversaturated/fuzzy
To balance the flubbing out, that's why popular boosts like TS9's and SD1s actually lose some low-end and they send just a bit of drive, lots of level (volume) and a more middy/bright tone.
So if you have a boost that can do some EQ-ing (like the MXR Micro Amp Plus, or a derivative of the famous Boss FA-1 ), you have more options of tweaking than with a mere single tone control like the SD-1 or TS9. Note: the aforementioned Micro Amp Plus and FA-1 will send a CLEAN boosted signal. The fact that a TS9 sends a dirty boosted signal, gives a different timbre to your amp's sound; in my experience, a TS9 can impart a sort of 'greasy' juicyness, which helps for lead playing, while the SD-1 stays a bit clearer.
Personally, I modified the hell out of a 90's RI DOD 250 Overdrive Preamp and just use that.