I think that how you go about making your music is different from person to person.
Personaly I work differently when remixing than when I compose my own stuff, mainly because a remix has to be "told" in a certain way, especially with C64 remixes where you want to catch the game spirit, but also the sound spirit of the original... I always try to hit both of these goals when I remix a C64 piece.
When I compose myself it's EXTREMELY improvised... I NEVER have anything in my head that I want to do, cause my main inspiration is SOUND in general... I simple fiddle around on the kaybord for fun, changing a few patches until I simply happen to play something that I find to be nice sounding... then I record this, place it in a loop, not more than a few bars.
Next thing, I continue to just have fun playing to the short sequence I just did... the idears come as they evolve by themselves... in the end I have a small loop going, usualy with bass, sequence and percussion... this is then repeated in the timeline for as long as I want the score to be... then I start to record melodylines, and make changes to the arrangement over time... putting in FX and breaks where I feel they should be.
So with my own compositions, I have nothing prepared before I start, and end up with whatever comes out of it, everything being decided in the creative process.
I sometimes do a bit of EQ in the creative process, at other times I do not... only if I can hear instantly that a low-cut is needed on an instrument do I use EQ at the creative stage, otherwise I usualy wait with this until the mixing stage.
For me, recording FAST and doing things FAST without spending too much time finetuning is VERY important... my experince has taught me, that what comes first to my mind is usualy the best sounding... if I spend too much time trying to analyse and fiddle around, I end up with something that is too complex, and the following day it just sound awful... it's like if you nerd around too much with it, you cannot hear the piece correctly wgain when you've had it out of your head for a while... it's like you cannot know the melodies in advance... you listen and you THINK what the next note will be, but then it does not... you have to sit and listen to the tune again for a period of time until you "know the melody" again before it start to sound ok again... but the next time you've had it out of your head, it's just wrong again. I believe this is what makes the difference in music wether it's an "instant ear catcher" or not... people don't like to be confused when listening.
With C64 Remixes it's all a bit different though... here I have mainly two ways I usualy go... one way is that I have an idear with the style, instrumentation and SFX etc... my remix of "Entombed" is one such piece. I like to give a Remix MOOD, and in other cases a specific genre ("Action Biker"/"Terra Cresta" comes to mind).
Another way is to simply just update the original to more pro instruments, but maintaining some of the sound... the instrumentation should have the same sonic quality as the original... "Demons First" is one such piece.
At other times I simply start out making some of my own compositions, and then suddenly find out while fiddling around, that a C64 piece fit perfectly for what I was doing, and these tend to end out pretty random... my first remix on the boards ever "Tiikerihai" is such a type.
So I guess you can all see, that for me, there is really no specific remixing rules I go by... it all depend upon what I want and what happens underways.
When it comes to C64 sound vs. pro instrument sounds in a remix, it depends on how much the oridinal sound is part of the "atmosphere" of the original... some SIDs will utterly fail in a remix, if you do not get VERY close to the original sound, if you cannot come up with something very clever in the arrangement... Try "Commando" for example... all those ringmoded and hardsync'ed sounds... Sanxion also... if you do not have some of that raw sound, it will in my opinion FAIL BIGTIME since these sounds drive the whole piece... the challenge lies in making these sounds on pro gear, and making them have the same feel, though hopefuly sounding even better!
But... sometimes it's just not possible or very hard to decipher how those sounds vere done, and simply sampling the original is the better approach... in my "R1D1" remix it did this with some of the ringmod'ed sounds, but effected them a bit... still retaining the original feel and sound.
Even though I actually have both a HardSID 4U studio edition, and a MIDI Box Sammich SID with the real deal SID chip, I never used it in a remix yet... a bit funny really... but maybe some day I will
Honestly... I've not remixed my favourite SID yet... and maybe I never will... mainly because the original sound so good to me, I don't know how to make it retain it's feel... it has all the challenges from above... synced and ringmod'ed sounds, very complex patterns of melodies. Whenever I try to start a remix of it, the original just still have something about it that I have yet not found out how to retain.... it's Hubbards "One Man & His Droid".
but who knows... maybe one day...