Multiband Compression Re-Explained*
I believe you have mis-intepretted what Multiband Compression is, it's literary Compression, in a Multiband format.
AKA; its Compression, compression is used on Basslines to round off the highs to then enable greater amplititude and grunt to the lower frequency's.
If you want to split frequency's, or would like to cross-over frequency's, then that is also possible in a Multiband Compressor, however the term squash would be incorrect, this is still compression, however with cross-over frequency's in place; which is seen as band blending/sharing.
-----
Metallic Property Enquiry Answer*
Metallic property's are often associated with oscillation or harmonics and are not generated by a Multiband Compressor; yet again i would recommend avoiding Multiband Compression, until you know how to control Compression for such purposes, as Multiband would then require that you compress or process the individual bands independently, but if you don't have a great idea of what your attack & release parameters are doing you might get lost and often be more damaging then beneficial... not to dis your ability in anyway, merely what i would recommend.
-----
Why (i think) you're getting that sound*
The Sound you are referring to is often an overly saturated sound, caused by an excessive amount of SQUARE waves generated by the red-lining nature of the synth; this is of course a guess, but the only way you would really get a massive metallic feeling, would be if your synth started redlining from the amplitude gained by the Multiband Compressor. Lets not forget that the Ableton Multiband has a +6db increase on it by default, you really don't wanna f' with that, and of course, the Metallic trademark would be the result of the limiting effect. It will sound fat in a pair of Dr.Dre Headphones, but like crap in everything else.
-----
Againsttimemusic@gmail.com
www.soundcloud.com/againsttimekaos
https://www.facebook.com/AgainstTimeKaos