Disclaimer: KRK is owned by Gibson Brands, which also owns Cakewalk.
The first time I heard KRK speakers was in a store and I thought they sounded pretty bad. I A/Bed with some other speakers and I expected all the speakers to sound at least vaguely similar, but there were significant variations.
Long story short: Most speakers have controls on the back for adjusting high and low frequencies. Make SURE they're all set to their default positions! This is
particularly true of the PreSonus Eris speakers, which have a ton of room compensation adjustments available. A salesperson having fun with those on a Saturday afternoon can make the Eris speakers sound like an audiophile's nightmare. Or consider Yamaha's ancient NS-10, which had a +5dB bump around 2kHz. I didn't like mixing on those but other people loved them because they'd undermix 2kHz and also hear a lot of midrange detail, so when translated to more accurate speakers, the mix sounded less "boxy" and the midrange balance was good. Yamaha came out with a modern version, the HS50M, which by itself has a much more accurate response (although
the bass response is somewhat light in the 100-300Hz range, with a boost around 70Hz according to Moozek) BUT also has a switch to give that famous 2dB bump. In fact, I think you'll find that linked review very educational in explaining why you do, or don't, want to mix on speakers that are or aren't flat.
Also, when comparing speakers you have to make sure the levels are matched. Some speakers are less sensitive than others and require being turned up more. There's nothing wrong with a speaker being less sensitive, but if compared to louder speakers - even if the levels are only a dB or two different - you'll lose highs and lows because of the ear's response, so the midrange will be louder and the overall sound will seem more muffled and "boxy."
Finally, choose speakers that are accurate. I won't mention names but I have tested quite a few speakers in my time. Several have had slight bass or high end tweaks or anomalies. If these are significant, you can end up under-mixing in a particular range if your room is acoustically up to spec because the speakers make you think that range is louder than it is. But if you mix in an untreated room, the bass will be all over the place due to the room acoustics - there will be build-ups and suck-outs. Using near-fields does take room acoustics out of the picture somewhat, but there's more influence than you might think.
How speakers are placed in your room, and the room itself, will determine the sound almost as much if not more than the speaker itself. This is why any speaker needs to be "learned" over time. Again not mentioning names, the [expensive] speakers I used before switching to KRK's VXT line were pretty flat from 50 to 500Hz, but had bumps in the lower mids and 10k, and a dip in the upper mids. Any two-way speaker is going to be lacking something, somewhere, so you learn to compensate...but some make you compensate more than others.
I've had the good fortune of working on some CDs in mastering facilities with monitoring systems costing tens of thousands of dollars, and which are about as good as is technically possible. When auditioning speakers, I bring CDs that were mastered in those facilities. Whichever speaker's frequency response sounds most like those CDs did in the mastering suite gets my vote, and it's not always the "best sounding" speaker.
I will add one final comment: being "in the business," I know which speakers have the best margins and which are running promotions to boost sales. Frankly these days, most speakers in the same price range are sonically and financially competitive, but given a choice between selling speaker 1 with a 40% margin and speaker 2 with a 50% margin, and if they're roughly equivalent in other respects, expect to be told speaker 2 is better