A little help about finding the right chord

[BertKoor]

2. Trintonsubstitute B7

tritone substitute
Tri-Tone: three whole tones. It wasn't named after a guy named Trinton or Tristan or whatever.
[/pedantic]

[jancivil]

However, the tritone subsitution does resemble the Tristan Chord rather closely.

Tritone Substitute is primarily about a substitute for a dominant (with a target tonic or tonicized other degree of a key).
So if "Bb" is our target, there is a ii - V - I implicated, and "B7" is substituted for F7, your basic "bII7" replaces "V7". (Originally the idea from Charlie Parker/John Birks Gillespie/Bud Powell et al was F7b5 is pitch-identical to B7b5 (Cb7b5); probably voicing-specific per root identity, ie., <b2 to 1> is basic.)

Again we're talking amongst ourselves regarding abstracting concepts from 'theory'.
We're coming from E? Whence 'ii - V - I in Bb' then? Were we 'on E' (in E? ) for a quick passing moment, or did we dwell? E to Cm, BAM?
This is all bullshit, incl. anything I'll say.

So why not: Tristan goes to Am, i in the key of A minor, through (what can be spelled as) an Fm7b5 for a moment, then a putative F7b5.
A great trick was played on the listener here: this sonority, if we want to ascribe it to a key in a normal tonal context ("I get carried away sometimes, being so normal and everything")

is ii7 of Eb minor. 180º off our target [A], or so you'd think.
But, to relate it to what's been said, it's like bII of V: the end of the progression is a nice normal V - i, so we've a substitute secondary dominant at the tritone. A century before bebop.

Technically it transforms a iv6 harmony into that thing. F A D to F A D#, the Italian augmented sixth chord. There's a convention by which Italian is made French by injection of the flat 5 essentially, let's call it B (I guess it's spelled this way because it's simpler?).
A G# appoggiatura (strong beat dissonance to resolve) makes the shocking F B D# G# (F Cb Eb Ab) 'half-diminished' (m7b5) sonority for a moment, but it's sorted in the voice-leading. F - "Eb" to E - D (just like in bebop etc), G# - A to A# for a nice #11 over E7 which resolves all normal and everything to B, then to the C of Am.

[Starbright]

I think we're going nowhere as long as we don't have the chord progression and genre at hand....

[BertKoor]

Fyi, the OP found a solution already, quite a while ago already
viewtopic.php?p=8997946#p8997946
But carry on if you must...

[jancivil]

"I put together another similar chord to A# of which I don't know the name but know it is similar, that translates better than not using it at all."
hence, we're talking to hear ourselves talk. I am anyway.

(next time, he's not going to have anything to go on, still)