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.

OP-

what brand sub do you use? Much will depend on the branding.
Otherwise, 80Hz is a good place to start tweaking to taste.

My crossover is set at 25 hz.  My speaker crossover is 55.  It does not sound like their is hole in the music.  In fact, the opposite.  Piano sounds full and real.  Voices are more intimate sounding.  I know that everyone room is different but I truly believe in almost any circumstance that I have had with sub's is less is more .  I can see why someone would not want one at all. I use a rel strata 3 currently.

My Maggies have almost nothing below 100hz.  I'm crossing over higher for them than any mini monitor I've ever used.

I used subwoofers for many years but could never completely eradicate occasional room node or crossover aberrations. As also noted by wolf_garcia, different recordings also contain quite wide variances in bass levels which can be extremely annoying. I do not want to constantly adjust settings on the subs to be able to enjoy music.

Several years ago when I heard a friends system using a DEQX processor & two subs I could not believe, firstly the clarity of the overall music, but mostly the absolute slam and transparency of the bass and no sense of room nodes or crossover point. I had heard nothing like it before.

I added DEQX to my setup early 2012 and, likewise I now have no issues whatsoever with twin sub bass integration. I have also created four configurations, selectable by remote, with 2-3 my ’normal’ listening setup and 1 for bass light music, 4 for bass heavy. In that way I can choose the appropriate setting and whatever type of music is played, bass is as near perfect as I could ever want.

That sounds like an interesting component.  

Nah, you just didn’t place the speaker/sub correctly. It’s very hard to do. On some odd rooms it can be  impossible.

Also, make sure you have bass management, and AVR or processor is really what you need, something like Dirac.

Actually, I do know how to set up a system and from my experiences, anything above 30 hz and systems start to sound more like a pa system.  The most important part for me in setting up a system is main speaker placement.  Then, where do I have my components.  Sticking a huge subwoofer in an area close to my other components causes problems.  Ideally, they would be on a side wall but that is a luxury in my view.  Subwoofers are made to reproduce low frequencies, not be a midrange.  If my speakers only go down to 50 hz, I cannot expect miracles.  I will never get the soundof a true full range speaker, but their are compromises there two. Most full range speakers are rather large objects so they need a larger room.  Speakers that go down to only 50 hertz are not made for a large room.  So a sub crossover set at under 30 hertz should give you more than enough low end and integrate fairly well with main speakers down to 50 or 55 hz.  

Actually, I forgot about room acoustics, that is probably the most important part of good sound