It's more complicated than 'high band = optimal, low band = not optimal.' The higher you go, generally it is more similar to SW, except that doesn't mean you're automatically getting terrific reception the higher you go. The X band here often is touch and go, even though the stations are 1 KW at night. My most dependable DX station in the X band is KFSG Sacramento (1690) followed by KBRE Merced 1660. KDZR in Portland (1640) is usually pretty strong, but not as strong as it was 11 years ago when sometimes I'd hear it into the late morning.
Paul is in Alaska (obviously), so his reception is going to be different from mine here in WA, due to his latitude, the sunrise/sunset times at his location, and other factors. During Winter he has more darkness. He's also right under the Auroral Radio Zone, and that may be affecting his MW reception as well -- either positively, or negatively (he could tell you which it is).
Propagation depends on your location, the location of the station, and the position on the dial can indeed make a difference, as well. But it's all the factors together that come into play. For example, as Paul mentioned, the night pattern can make a difference. He gets KVRI Blaine 1600 like gangbusters because they basically beam all their signal in his direction. I get them most nights, sometimes really well. Other stations he gets really strong in AK I don't get all that well at my location.
KMIK 1580 Phoenix, when they were Radio Disney, were heard in Australia a lot, apparently, because their 50KW night pattern sent maybe 200KW out in the direction of the Pacific Ocean. Certainly, the high position on the dial probably helped, but when you're pushing your signal in one direction with 200KW ERP, it's probably going over the Pacific no matter where it is on the MW dial.
The most dependable California stations here in the Seattle area (at my location, anyway), are KGO and KFBK, followed by KNBR and probably a weak -- but generally consistent -- KFI. XEWW 690 is marginal most nights, covered by CBU (which is 140 mi N of me). Then you've got the decent regionals, which often show up, but aren't consistent, like KSTE 650.
I think most West Coast DXers would give KGO a better rep for being heard up and down the coast than XEPE or even some of the other high band stations.
XEPE came in much, much better in the 2010s here than it has over the past 8 years or so. I used to get it as late as 9 a.m. during the Winter and Fall, now it's grainy year round, if it's there. XEPE is in Tecate, maybe 120 miles inland from the coast, so they may have different skywave characteristics than they would right on the coast. I think the current Solar Cycle isn't yet favoring the type of DX that the last one did. It has yet to hit its peak. So that definitely comes into play as well.
The lower you go, probably the better the groundwave propagation. And if a station has a rep of getting out well -- like the two I mentioned, KOAC 550 and CBK 540 -- they'll get out really well at night, too, via skywave -- once that skywave kicks in. KOAC is only 5 KW, but they are one of the most dependable low band stations here in my location.
Generally, skywave will kick in on the high band before it reaches the low band. The propagation chracteristics seem to change in the 900-700 range.
