Merely flattening the response shouldn't have an effect on "speed" or "articulation" of the bass response. You will notice that with a flat frequency response you will typically find the lowest and highest frequency ranges quieter than those more in the middle range. Again, that has nothing to do with "timing".
Reflections in a room being cut, whether by a filter or just a better design, will not change the "speed" or "articulation". You still are doing nothing to shift the frequency forwards or backwards in time. Shift probably was the wrong term for you to use, but it's not a minor nitpick as far as the mathematical terminology goes. Plus shift is correct as far as an amplitude shift (up or down, not forwards or backwards).
Your very last sentence is correct-in that moving speakers or subs around will alter frequency response and "articulation" (although I think that's still an incorrect term, I think your original phrasing of "speed" is more adequate here). The reason it's correct is that reflections, standing waves (peaks/nulls), and direct sound all play a part in what we hear from a speaker. Moving a speaker will cause those parameters to change (I'm thinking, and will gladly be corrected if I'm wrong, that the direct sound radiated from the speaker will have little to no difference in it's frequency response from this scenario. However, I know that the other parameters will greatly change and affect what we hear). As far as the "speed" goes, that will "change" only because the physical distance between you and the speaker changes. The sound will either have to travel farther (more time=slower) or less far (less time=faster). Also factoring into that would be the reflections-their distance traveled will also change, thereby slowing or speeding up the time it takes for their sound to reach you vs. the directly radiated sound. This is mostly likely what results in a blurred, muddy sound (inarticulate for your phrasing?).
Frequency response however still has nothing to do with the articulation/speed/whatever. That is how, in the original post, I posit that your friend is more correct. You aren't completely wrong, but the basis for what you hearing is wrong. You have your cause/effect relationships mixed up imo. Frequency response /= speed and articulation. Room placement can and does have an effect. This is why room treatments are often discussed. They not only help level out frequency response, but in some applications could probably lower it so much that for a reflection as to eliminate (for all practical-ie audible puroses) that source of sound. Thereby potentially fixing or improving the impression of "speed" or "muddiness" or "articulation". I use the quotes not to mock, but just to point out terms that are subjective and difficult to quantify.
Denon 2807, Samsung UN55D6900 55" LED 3d, Dali Ikon 6's, Vokal 2, and Ikon 1's, and a LFM-1 EX Subwoofer from Outlaw Audio.