I'll give it a try...
Many modern bass amp designers have discovered that high pass filtering is very important to the sound and feel of an amp, and to the longevity of speakers... ESPECIALLY as amps get more and more powerful. Speaker mechanical power handling is not a simple number as the specs might lead one to believe. It's a series of numbers that generally decrease as frequency decreases. A speaker rated at say 500 watts (mechanical limits) at 1kHz might take 450 watts at 100Hz and 400 watts at 40Hz and 200 watts at 30Hz. Each cabinet/driver combination has a different set of numbers depending on the driver and the tuning/interaction of the driver with the cabinet.
It's up to the designer to consider these limitations and to be responsible to the player to avoid unintentionally overdriving the speaker (within reasonable conditions) with the proper application of compression/limiting and high pass filtering.
This is something that us pro audio designers learned years ago (that's the industry I cut my teeth on and still work in somewhat as a designer) and is standard on just about every system processor and amp on the market today. Otherwise there would be a heck of a lot more blown drivers out there.
So, for bass guitar players, the high pass filter must be somewhat adaptable to situations where both large cabinets (with extended low frequency loading) and the new sub-compact cabinets (less low frequency extension) are used somewhat interchangably. The LF Extend filter network on the Shuttles allows a small shift in the high pass network (LF Extend engaged lowers the HPF corner frequency) but also changes other parameters in the filter's response since it is not just a HPF. It's part of the feel of the amp and contributs to the voicing and playability on a wider range of cabinets. It's not uncommon for a player, for instance, to take a 6.0 and play it on a 410 or 810 at a large venue but then take it to church and play it with the STL-10 compact cabinet. Much better results will be experienced with the 810 using the LF Extend engaged as the full(er) low frequency response of the amp will be available to the cabinet that will benefit from the lower response.
I am reluctant to go into any more detail because things tend to get confusing and then taken out of context.
Hope this helps.