I had a quick look at the manual online, and found this, relating to the “MIDI Program Select Mode” parameter, on page 14 of the Global parameters:
PAGE 14 — MIDI PROGRAM SELECT MODE
This parameter determines how your QS6.1 deals with MIDI Program Change Commands. There are three possible settings:
OFF turns all Program Changes off. With this choice selected, your QS6.1 will neither send Program Changes nor respond to them.
ON makes the QS6.1 respond to incoming MIDI Program Changes, and send them out whenever you change Programs from its front panel. In Program Mode it will send and receive Program Changes over the currently selected MIDI channel. In Mix Mode, Program Changes are received over all active MIDI channels, but sent only from those displayed channels that have had MIDI OUT enabled. Please Note: With this setting, incoming Program Changes will only affect the Programs associated with each specific channel, not the entire Mix.
MIDI MIX SELECTION: CHANNEL 1 through CHANNEL 16 are identical to ON as far as Program Mode is concerned. In Mix mode they work quite differently, however, allowing you to change entire Mixes in response to Program Change
commands. Select CHANNEL 4, for example, and a Program Change 22 command coming in over MIDI Channel 4 would automatically select Mix 22 in the current Bank. The same is true of Bank Select messages which come in on Channel 4 in this scenario; they’ll call up a Mix in another Bank. (Program Changes coming in over other MIDI channels would continue to work as before, changing any Programs associated with those channels within the current Mix.)
This suggests if you set it to MIDI MIX SELECTION, and you’re in Mix mode, then program changes will switch between different pre-programmed Mix setups. So that’s half of a solution - you could simply always use Mix mode, even for songs without layering, and just setup Mix patches accordingly. That’s roughly what I used to do back in the days when I used a workstation synth live.
However, if you’re adamant that you want to switch between Program and Mix modes on the fly, you may need Cantabile to send sysex data to the Alesis, but I can’t find any reference online about whether the QS6.1 allows this to be controlled remotely, or any information on how this might be done via sysex.
Many synths simply designate some banks as Mix (or their equivalent) mode banks, and others as Program mode banks, so you could switch between modes by selecting different banks as well as program numbers. I know the Yamaha Motif series use that approach. It seems the QS6.1 doesn’t, unfortunately.
Another approach harnesses the power of Cantabile, by setting the QS6.1 to local off, and sending your whole keyboard’s range out to Cantabile. You can then use Cantabile’s powerful filtering/zones and song states to send MIDI back out to the QS6.1 to play sounds on your various Mix mode channels. That might give you the best flexibility because you can then also use VST plugins alongside the QS6.1 sounds in layer/split setups.