Obviously, as with any kind of question like this, there's bound to be differences in the way people experience these states and in the way they describe their experiences. Sometimes, they may be talking about the same thing, but using different descriptors that have personal meaning to them, but may not translate in the same way to other readers because they associate a different interpretation of the descriptors. Keeping that in mind, I will attempt to communicate a response to your question.
I think you kind of answered the question yourself within
the question you asked. You asked: "...how can we differentiate between our concentration becoming more focused
and consequently a quieter mind
and lapsing into a numbness
that just feels good?" The way this question is worded already shows an awareness of a difference.
However, if you need more details, Thanissaro Bhikkhu once wrote an excellent essay on this very issue entitled Jhana Not By the Numbers
which at one time helped me to sort out this very same question. Over the years, I have found his descriptions of the practice of meditation to have been confirmed by my own personal experience. His writings, therefore, because they are so prolific can be a valuable resource for figuring out these difficult questions.
A few pertinent excerpts from the essay might help to shed some light on the matter. Talking about his preceptor, Ajahn Fuang, Thanissaro mentions: "As he once told me, 'If I have to explain everything, you'll get used to having things handed to you on a platter. And then what will you do when problems come up in your meditation and you don't have any experience in figuring things out on your own?' "
"So, studying with him, I had to learn to take risks in the midst of uncertainties. If something interesting came up in the practice, I'd have to stick with it, observing it over time, before reaching any conclusions about it."
There is a decided qualitative difference between a mind that is dull and basking in bliss, and one that is alert, aware, awake, sharp, and focused on its object (or whatever arises in the mind, as it often the case). While the former state is, as you state, a "feel good" state, the latter state not only feels good but is rock solid in its execution and stillness without devolving into a dull or unfocused pleasant state. One of the hindrances that produces this dull state of mind is the lack of application of mindfulness
to the task at hand. Mindfulness keeps the mind alert, awake, and sharp, and helps produce that rock solid "feeling" of a focused and concentrated state.
As Thanissaro writes: "Strong concentration is absolutely necessary for liberating insight.
'Without a firm basis in concentration,' he [Ajahn Fuang] often said, 'insight is just concepts.' To see clearly the connections between stress and its causes, the mind has to be very steady and still.
And to stay still, it requires the strong sense of well being that only strong concentration can provide."
Bottom line: dullness of mind is cured by developing and asserting strong mindfulness on the object at hand. This has a very different feel or sensation to it than the "feel good" state you mentioned. And once you experience it, you (hopefully) won't want to return to the "feel good" state.
As Thanissaro writes in the essay about hitting a state wherein he lost all sense of the body, internal or external sounds, or thoughts or perceptions:
The second state was one I happened to hit one night when my concentration was extremely one-pointed, and so refined that it refused [to] settle on or label even the most fleeting mental objects. I dropped into a state in which I lost all sense of the body, of any internal/external sounds, or of any thoughts or perceptions at all — although there was just enough tiny awareness to let me know, when I emerged, that I hadn't been asleep. I found that I could stay there for many hours, and yet time would pass very quickly. Two hours would seem like two minutes. I could also "program" myself to come out at a particular time.
After hitting this state several nights in a row, I told Ajaan Fuang about it, and his first question was, "Do you like it?" My answer was "No," because I felt a little groggy the first time I came out. "Good," he said. "As long as you don't like it, you're safe. Some people really like it and think it's nibbana or cessation. Actually, it's the state of non-perception (asaññi-bhava). It's not even right concentration, because there's no way you can investigate anything in there to gain any sort of discernment. But it does have other uses."
Read the essay (and anything else that Thanissaro puts out about meditation) you won't be disappointed. He knows what he is talking about and can become an excellent source for testing one's own experiences against.
"The gift of truth exceeds all other gifts" — Dhammapada, v. 354 Craving XXIV