Digital BW, The Print

So I'm new to this but am now at least knee deep in seeing what is 
possible w/ modern inkjets for b&w.  At the moment I have an Epson R280 
loaded with the 6 cartridge Eboni inkset from MIS.

I shoot two films, Ilford HP5 and Delta 3200 in both 35mm and 120mm. 
I'm quite happy w/ my film and processing methods.  Scans look great. 
Traditional wet prints look great.  Some of my new inkjet prints look 
great, others look very bad.

As a generalization it seems like in some circumstances and with some 
photos the printer is able to convincingly render the film grain and 
it's gradations smoothly and in other cases it can not.

Where I notice this most is usually with people's hair or beards.  They 
often appear very splotchy as if the printer is printing straight black 
or white and doesn't understand that there should be a gray transition 
in between.  These are mostly natural light candids on 35mm where grain 
is fairly visible.  I like grain.

What confuses me somewhat is that if I take the identical file and print 
it out with QTR in Black Only mode, QTR seems to handle these 
transitions in tone much more smoothly and hair is rendered much more 
convincingly as opposed to splotchy. I would just print with QTR but I 
do not like the brown look and I greatly prefer the overall tonality of 
the 6 cartridge setup.

So, if the above makes any sense, am I running into a limit of the 
technology or am I doing something wrong?  Why is QTR rendering hair 
much more convincingly using only one cartridge?  What should I be 
experimenting with?