I 100% of the time use the stock black color in CMYK in Illustrator and print using "MAX IMPACT" ink profile my blacks come out SOLID on SP300V, versa works, oracal 3164 media, oracal 3164 media profile. Exact same results using 3165 media profile on 3165 media using MAX IMPACT.
If you want, I can send you an eps file I would typically use and tell you exactly what settings to use for versa works. If it doesn't come out nice and solid black THEN, then you have an issue. You might just not be setting up the FILE ITSELF correctly...
I would bet it's the cable that connects the head carriage, if it even is an issue with the printer. They're a wearable item on the SP300 - which is why the SP300i got a different cable that doesn't bind up and break all the time. I hear my cable binding from time to time in the printer but there's nothing I can do about it.
PS i'm just learning right now too. 1.5 months into printing on SP300V and love it so far. Learned a lot from these posts BTW (THANK YOU GUYS!) There's not a whole lot of info on the Sp300V out there that I was able to find. Gonna try some new things suggested here myself and see what happens with my typically grainy photos