So...
I purchased Mike Seniors book. (Mixing Secrets for the Small Studio)
Much of it I was aware of/had found out by myself.

For example, I have treated my room quite well (GIK Tri-Trap bass traps,GIK 242 Panels and some Auralex foam at first reflections)(GIK Soffit trap soon to be added). I have a couple of sets of okay reference headphones (Sennheiser HD280's + Beyer DT 880's). I have an Avatone Mixcube. I have taken room modes & various other things into account as much as I can. I thought I had a pretty good handle on the subject.

One thing I was completely unaware of was how ports on monitors can drastically twist what I hear.
I have a pair of KRK Rokit G5's positioned nicely on some good stands.
After reading the very beginning of Mikes book I went to my monitors and stuffed some cloth into the ports to see what difference would be made.

I'm a little freaked out by how much 'loss of bass' happens :/

How am I supposed to know if I should eliminate the ports by stuffing them, or leave them as they are? How can I figure out what will give me the most accurate response?
I realise this will be unique to each situation, but what can I look into to get a decent of idea of how my room is performing?

I have previously had to work in absolutely rubbish rooms with horrible equipment, now that I have the option to spend more on equipment I want to try get it as right as I can.
Different monitors won't really be an option right now (Unless I can sell some of my redundant mics etc )(Go buy some, I made ads) . Anyway....

Thoughts?
Better monitors? Or deal with what I have and stuff/don't stuff ports?

I think the answer depends entirely on the monitors in question, and also depends on what you want from the monitors.

I recently purchased a very inexpensive and very small set of powered speakers that are receiving a certain amount of attention for a portable down market solution in some applications.

I could clearly hear some resonances, and came to the following conclusions:

1. Plugging the ports lost about 1/3 to 1/2 octave of the lower end, made the speakers a little less efficient, but made huge improvements in every other regard. (Reminder-this is very down market gear!)

2. Some of the resonances I heard were also heard on "real" and much more expensive monitors (even sealed cabinet ones)-but they seemed less obvious on these other monitors as the resonances were broader and thus a little less identifiable on these other monitors. It also seems that in my own case, the resonances were a part of the program material and were simply handled differently by different speakers. OTH, there are many speakers than bring their own resonances to any program material.

3. The position of small monitors makes a huge difference. Moving them a few inches from the lip of a surface or other little moves can change everything.

4. Ports can make a speakers behave quite differently at different SPL's, so that has to be taken into account as well. Generally, I find I need to play ported speakers a little louder than I'd like.

The advantages/disadvantages to plugging ports seem to be very situationally specific.

And a reminder of the caveat in my own case-the speakers were down market and were not intended for any sort of serious monitoring.

You could avoid the whole issue by getting sealed cabinet speakers....you'd have fewer options to entertain!

Or keep what you have and use them both ways. Why not?