I am new to using samplers. Can they play MIDI files dowloaded from the web. How many songs can they normally hold? I currently have a Roland Juno Stage that play up to 99 Midi files. It is sweet to play with. However I want something smaller that has just as rich as sound using raw MIDI files.

I don't own a 555 but I would be very surprised to find out that you can import MIDI files to it. My 404 certainly can't do that.

but I'll bet you could probably find a little yamaha sequencer that will...

pretty sure the only kinda files you can import onto the 555 are .wavs, least thats what i do anyway. 555 does have MIDI though

The SP family will not satisfy your desire for rich synthesis as they do not have any. (the large 606 and 808 have some but I would not say "rich")

Cannot think of anything besides software on a tablet that would be small size and play MIDI files and have a good synth engine.
There are boxes like the Korg ESX etc that have sampling and synthesis and may play MIDI files. Let us know if you find something that fits the bill.
C

No MIDI file playback but worth mentioning- I would LOVE to own one of these tiny monsters:
http://www.teenageengineering.com/products/op-1
C

you can make rich synth sounds with the sp 404 303 .

just need lots of resampling reverb and FX and timestrech.

then put it in a sampler with sample tuning like real pitch and thats a cool way to make synthesizer sounds.

I used to use the sliders on the mc505 to control the pitch , it can make real awesome cinematic phrases,

or open any kind of file as raw data into audaicty, usually it is noise . but sometimes you will find wicked cool beeps and glitches. try opening it at lower sample rates so it plays slower. then sample those with FX and youll have all sorts of awesome sounds to make synthesizer notes with.

especially on the 555 or 606 where it has a synthesizer, but they are very buzzy, and the 606 uses a triangle instead of a sine wave.

so its hard to get the 555 and 606 sounding deep and rich , but if you have a 202 or something you can filter your synths .

a cool signal chain to capture synth phrases around something is.

synth > 202 (capture synth line) > 404 or sample track drums playing > speaker

then something like bounce the sample trak drums to the 202 and match them together. or make the song in the 404 sample trak but 202 in the middle is usefull for catching live things and the drums further down the signal chain, so the loop of the 202 should match.