And your ears can hear absolutely no difference in a blind test
Indeed.
[Please recognize that this tube has not been burned it yet, but my initial impressions we based on the idea the tube had been burned in by previous owner. It's almost NOS.] Alrighty. Been listening to my Philips Miniwatt E188CC SQ for a few hours now.

I really enjoy this tube. First impression: "eh, this is alright". I was not initially terribly impressed with the tube for whatever reason. I decided to listen for a few hours to get a better feel of it. After that I put my Genalex back in to see if if there's a huge difference because I didn't really notice one initially. I have to say that as good as the Gold Lion is there is a pretty substantial midbass hump and even some muddiness that's hard to miss now that I've been using the Miniwatt. Also Gold Lion still has a bit of a sterile, digital sound. Too analytical for my tastes.
This Miniwatt SQ tube does so many things right. There's a little bit of tube-y harmonic excitement in the highs, a little more shimmer than the Gold Lion also sparkle is more present here. Nothing like the 6922EH though. The Miniwatt highs are so much more realistic sounding than either the Gold Lion or EH.
The Miniwatt's bass is substantial and very well extended and balanced (Listening to edIT - Battling Go-Go Yubari in Downtown L.A. at the moment and there's definitely punchy full bass here).
The mids are pretty much spot on. A little bit more forward than the Lion. The level of detail is very nice as well. I definitely find this tube to be more musical than either of the previous tubes. Very realistic sound.
Summary: At first I wasn't really terribly impressed with this tube but I'm definitely appreciating it a lot more now that I've done some more A/B and learned it. One of the things that the HD558 is known for is recessed low-low end. I find the subbass to be more present in this tube than the others. Midbass is very nicely done. Not overly loud. Tight and full. Mids are present and nicely up front. Highs are present, fast, shimmery, and very high frequencies tend to roll off a little bit but not excessively. Enough to say that they are not fatiguing at all.
TL;DR? I love this tube. Definitely going to be used more than the Gold Lion although I see times when the 6922EH will still see some use. Especially with electronic music and some classic rock.
(OH MY GOD NP: Skrillex feat: Sirah - Kyoto is massaging my eardrums...... The bass is definitely tight, accurate, and very full. Roll call. Mr. Bass? Calling Mr. Bass. Oh... yes... There you are!)
Tomorrow my Amperex Gold Globes come in.

So do my Soundmagic HP100's.

Now back to work for me.....
Edit: I removed the inline resistor for this review, by the way, as I found the tube to be very muddy with it. On the other hand, it helped the other two tubes' signature significantly.

I shall perform more listening to the tube tonight.
Edit 2: Apparently this tube only had 1 hour of burn on it when I bought it from the seller. That would explain why I wasn't exactly blown away by it initially. I was expecting a mature tube. Either way I am definitely enjoying listening to the tube and I'll be burning it in overnight as well. It's really turning into something quite special. I told my dad to listen to this and the Gold Lion and he said that this thing was much more realistic not knowing anything about the tubes.

"That little piece of glass cost $50?" "Yeah it's normally around $150-$175. It's 40 years old." He gets this huge smirk on his face with his lips pursed and just says "Oh." I think he likes it.