I had a C35... was good little unit, only replaced it when I needed something for larger negs!
What your experiencing is correct regarding the exposure times. As you add filtration, the amount of light getting thru decreases. Happens both for yellow and magenta, so that parts normal. When I moved to VC paper (I originally used graded paper) after initially using the inbuilt filters I changed to the drop in multigrade filters as the changing times used to annoy. I never tried the dual filtering method (this was before the internet and I had minimal books to learn from and none of those mentioend it). The beauty of the MG filters is they are the same speed from 0-3.5 then 1/2 speed (double your exposure times) for g4-5. Makes life easy!
Now to the contrast issue. There are thoughts that colour filtes fade over time and become less effective. I haven't experienced that myself and I've got some very old MG filters that match some new ones I bought after reading about this on the 'net! If you can do a decent contact, then you should be able to produce a decent print. The neg must be ok. I'm wondering if your not adding enough exposure when you alter the filtration to compensate.
Since you state your a newbie, here's a basic printing concept to guide you... Exposure governs the highlights, filtration governs the contrast. So, when doing your test strips, base your exposure on the highlights and alter the contrast (how dark you want the blacks) with filtrations. This is where the matched MG filters come in handy as you can change filter and not have to re-run the test strip to get those highlights right again.
Another thing is to make sure your developing long enough. What paper and print developer are you using? A popular method for determining the print devellopment time is 6X the emergance time. When you see the image starting to become visible, multiple that time by 6 and make that your full development time. i.e say it takes 15secs to emerge, make full development time 15*6=90secs. As the developer ages, you should extend the time to suit.
My other thought is what globe you have in the enlarger. The original MELAMP 55 globes are hard to come by. I ended up putting a dichloric globe in mine as I couldn't get the original globes at all (here in Australia)
So, here's what I suggest. Use the drop in filters to ensure you can get some contrast changes in your prints.
1. Make a straight print with no filters. In theory this should be equivalent to about a G2 filter.
2. Make a test strip with the G2 filter in the drawer and no inbuilt filtration. Use double the exposure of print 1 as a guide of where to base your test strip times
3. Make a G2 print with the time that the highlights match Print 1
4. Make a G3 print with this time
5. Make a G4 print with twice the G3 time.
If you don't see a difference, then something is wrong.
I'll leave it at that for the moment... once you've tried that we can see what the result is and proceed.
Someone else might have some other ideas too...