Keep things in perspective here.
A dip in the mids doesn't mean it has an intolerable lack of mids.
It's going to be a personal pref/gear thing just like with any other product.
Just watch the vids, read the posts, but take them ALL (including mine) with a grain of salt. Remember, these are ALL largely just "IMHO/IME" even if folks don't explicitly say so.
+1 I just finished spending time with a TBers new LDS 15/6 (NICE) and the Streamliner and F500 with the P and 60's J. All sounded very nice.
Interestingly, I noticed very little max volume difference between the Streamliner 900 and the F500 at 8ohms. Both loud, full and punchy. That could be a voicing thing.
With the P... forget about it... for my ear (and Don's also), the Streamliner was just fat and perfect. The best tone we achieved was with the Streamliner flat except for a touch of treble reducition (treble at about 10 o'clock). Using Don's words 'now, that is a P Bass'. Perfect.
To Vic's point about the mids, I GREATLY preferred the F500 with my Alleva-Coppolo 60's J and the 15/6. There was a bit of hollowness in the low end of the Streamliner with the 15/6 down low with that bass that I couldn't dial in with the low mids. Still sounded great, but I missed the burp and low mid punch of the F500. Of course, part of this is the wide, smoother 15/6... I didn't feel the same way with the AE410.... so 'mix and match' continues to be key. This reinforces my feeling that for some, a tight 'tube friendly' cab will be key to enjoing this head. IMO there.
I could get the F500 to sound pretty similar to the Streamliner (not so much in the tubey thing, but in the general tone profile) by cutting the treble to 10 o'clock, cutting the upper mids at around 1K to 10 o'clock, and then engaging the VPF at 9 o'clock (which softens the low mids and expands the deep low end.
So, to Vic's early comment, and to others who hear a 'mid scoop' if you have a very even, big sounding cab like the 15/6, you will get more round low end than 'burp and punch', and it is hard to dial that in with the Streamliner.
I cotinue to love this head with my P Bass, and have yet to hear a cab combination that to my ear sounds better with a J than being driven by my F500 (which is more similar to the Shuttle voicing). IMO there.