You are hereConnecting MIDI Hardware for Playing Live
Connecting MIDI Hardware for Playing Live
"...I got a roland mc500 sequencer, an akai s3000 sampler, a nord modular keyboard, an access virus a synth, a roland r8m drum machine with 909 soundcard, and a 12 channel mixer...."

Slave To The Rhythm
"... I would like to be tweaking the melody and basslines live using hardware and midi for the best sound, but I'm not quite sure what the optimal MIDI setup would be. Any comments or suggestions this would be highly appreciated."
So, you prefer all seperate hardware for live? O.k. you are a braver man than I...
Based upon 15 years of making records with synthesizers and having come from a studio full of seperate hardware boxes (there was no alternative back then) I honestly think that these days hardware offers absolutely no sound fidelity advantage over VSTs - assuming that one uses quality VSTs.
Maybe a few years ago this was not the case, but the technology and algorithms have matured and computer power has increased dramatiacally in the interim.
In my view, hardware's advantage is primarily the dedicated user interface - which btw is a major advantage.
...Do you have suggestions for how to handle all the MIDI power that it takes for high tempo, multiple track psy in a live setting?
Well personally I wouldn't go for external hardware triggered via an external MIDI interface for this very reason.
For optimum sequencing MIDI "tightness" then using VSTI's inside an audio/midi sequencing environment with real time control accessed via an external dedicated MIDI controller is your best bet.
Sequencing external synths via MIDI will always add some MIDI jitter. Midi is fine for external realtime control, say tweaking envelopes or filters with an external midi controller, as well some and other functions. But for triggering percussion on high tempo rhythmically precise music such as psy trance, then MIDI can begin to show it's age. Especially when compared up against the newer VST technology, which is sample accurate with regard to timing.
And once inside the VST host environment, sample accurate translates to absolute accuracy, given that all the rest of the sounds are referenced against the same timing reference "ruler".
You can test yourself this with a VST host such as Cubase, Ableton Live etc. by triggering say a dry snare drum from a vst sampler via a midi part. Then, bounce the track to audio and bring the audio back into the sequencer alonside the original.
Then, invert the phase of the bounced part (with a wave editor like Soundforge etc.) Then loop and play both part together and adjust the levels to exactly match. The result will be silence (!) as one wave mathematically cancels the other out. However, I digress...
To return to your question of how best to set up your external synths when playing live: first of all you do know that you can run all of your external sound sources with just your one midi output?
You may have to do a bit of planning, deciding which midi channels to use for what, but it can easily be done.
You can do this by assigning the sixteen MIDI channels of that one MIDI output between your devices. This would also be the simplest set up for what you want to achieve.
So with just the one MIDI output socket, assign the channels for example thus:
Midi channel 1 for MPC drums
ch.2 for samples mapped across a keyboard
ch.3 for Virus synth lead 1
ch.4 for Virus synth lead 2
ch.5 JP8000 bass synth
etc.
If 16 channels are enough, but you don't have a MIDI thru facility on all your synths, then the one way would be to get a MIDI splitter box like this one:

I recommend this one* because it needs no power as it uses the midi line for power, so no batteries or AC adaptor to worry about. Also it's small and light for travelling, and it's cheap enough. I've had a couple of these in my studio for at least ten years and they just work.
Alternatively you could "daisy chain " midi devices if you do have a MIDI thru on all your boxes like this:

Daisy Chaining MIDI Devices Together
I hope that's given you some ideas. Best wishes and good luck with your live playing.
Cheers,
Billy Cosmosis
*More info at: http://www.philrees.co.uk/products/thruunit.htm#v4