subwoofer crossover- lowest is best almost always?

Sounds like you have a good ear. :) Your approach is pretty sound, but only some one with pretty good ear training would normally approach it empirically. 

You could be running into a couple of issues.  First is "room gain" The phenomenon that the room extends the anechoic -3dB point of the speakers and boosts the bass overall. Getting an extra 10-20 Hz bass extension  is not impossible.  The second is room modes. If you end up with a bass peak in just the right region it can also either extend the bass, or make it feel bloated and exaggerated. 

I would encourage you to measure and see. The Dayton iMM6 is under $25. Together with Room EQ Wizard (free) you'll get a much better idea. I use OmniMic but it's around $300, but it's very easy to use and, for my speaker projects, ideal. 

Measure, traps, miniDSP FTW! :) 

Personally, I'd always try to cross a sub at somewhere between 70hz and 80hz or so.  Note: I use an active cross-over and this observation may not apply at all to those who don't.

Getting the kick-drum fundamentals (typically 50-60 hz) directed to the sub has two great benefits, IMO:

1) A decent sub is typically much better suited to moving this volume of air and will do a better job of it than even an outstanding main loudspeaker.

2) Relieved of moving that much air, most main loudspeakers will sound better thru their entire bandwidth.

Per Erik's recommendation, using Room EQ Wizzard (or a variant) will help you dial in smooth response - that's true even if you're crossing higher.  Even better, IMO is digital room correction and bass management (Audyssey, etc).  Not audiophile approved, but a great solution for integrating a subwoofer IMO.

Too many factors affect where the crossover should be. IMHO, any blanket statement is not valid.

I'm too egocentric to allow digital room management (I'm a live sound tech and an Old Person) for my 2 subs, but luckily the shape of my room (high sloping ceilings with impeccably tasteful furnishings) works well to keep standing waves from polluting my brain…only in the sweet spot however, proven when I've been plastered to the ceiling although I might have imagined that part…in any case one thing I find hard NOT to notice is the fact that all recordings sound different regarding bass response level, and I simply have to turn the subs up or down a little sometimes. That said, I think it's a good idea to change the crossover point from time to time just to calibrate your brain…you can see why it's set at a certain frequency in the first place, or maybe why it shouldn't have been. I tested my main speaker's frequency response in the room with a meter to see what's what and at least initially put the subs at around where the main's bass drops off substantially, but changed to a little lower point as it sounded more accurate (whatever THAT means) to my addled self.

The key is TONAL BALANCE.
Your system is bi-amplified by default using an active sub so the sub volume and crossover point should be set for maximum tonal balance meaning when you can pleasantly listen at widest volume range possible.