From your post, I would agree that you should look into W4.
The W4 is really an engaging IEM that sports a non fatigue listening and sounds so musical in its tuning. I would say the amount of bass the W4 throws out punches deeper than the GR07. It goes pretty deep and tight. The bass is pretty fast too with a slight decay. It is tuned more towards quality bass rather than quantity which makes it really enjoyable. A slight mid bass hump is also present which gives this IEM leaning more towards the warm signature.
Mids are very well positioned and I can safely say most people will definitely enjoy the mids on the W4. To me, it has been the best mids I've come across on all the IEMs I've heard or owned. Mids on the W4 are very slightly forward positioned which gives vocals a very lush and smooth sound. Both male and female vocals sound full and energized which is really a joy to listen to. Instrument separation is extremely good as well with distinctive placement and layering of instruments. Clarity wise, it's pretty good as well just that I do find a slight veil hindering the clarity in the lower mids.
Treble on the W4 is nothing short if fantastic. It is extremely linear, smooth and non fatiguing! Even pumped up high there's no signs of screeching or painful treble.
The only thing that I find, is the W4 requires a certain volume to come alive. Lower volumes will result in mild bass/sub bass and the vocals does not come across as that dynamic.
Note: I paired the W4 with silver dragon cable.
The Heir Audio 4ai I would agree with Tinyman that they will not fit your bill nicely. Bass response/quantity is lesser than both W4 and GR07 to my ears. However, that does not mean it has a lame bass response. Comparing to SE535Ltd, bass response on the 4ai is still much prominent.
The bass on 4ai punches deep, tight and really fast. It does not have a mid bass hump. Bass only kicks in when the music calls for it. If bass is present in the music, the 4ai portrays it really well with fast thumping bass.
Comparing the mids of the 4ai to W4, I would say both are good but the W4 edges out more with a fuller vocal which is lusher. Vocals on the 4ai may sound abit thin compared to W4. However, clarity on 4ai is better as there's no veil hindering it. Instrument separation edges out on W4 with amazing separation on par with UM3X.
However, I do find the timbre on the 4ai abit unrealistic sounding. Cymbals sounded artificial. Timbre on the W4 sounded natural and good.
Both W4 and 4ai has a pretty wide soundstage about 4th row from the theatre.
I've heard the UE900 too. IMO, it sounds pretty much alike to the W4 but is thinner sounding and bass is not as deep as W4 to me.
Hope this helps!