Let me preface by saying I don't care for 15" in a sub for the average home living room/HT space, just totally unnecessary IMHO. The drivers I will speak of are the 12" versions of all the drivers discussed. Now, having said that:
Having heard BOTH the HF & Titanic, I agree w/deathinc. The HF/HO, while not the same amount of output, DO sound noticebly better for clarity & control when compared with the Titanic. This is the reason I bought my first HF instead of the Titanic. And still have it. The HO (which I also purchased), certainly is no slouch either and why I bought one also.
And as for "proof"
, this is the internet, anyone can say anything and come off sounding knowledgeable. However, in my case, I literally live less than 30 minutes from Parts Express in Ohio and am a frequent 'visitor' there. So I have an idea of just how their drivers sound.
Now, for the horrible advice you gave for going with a 300 watt amp instead of a 500 watt amp because a 500 watt amp wouldn't be turned up past 1/2 way, is just ludicrous. I am running 500 watt plate amp with the HF (and HO as well) and it is absolutely perfect. Ample headroom for those times when we want the audio cranked up, and subtle, but noticeable at 'normal' and lower volumes. The HF just purrs thru it and we couldn't be happier.
The Titanic has a well deserved reputation, this is not arguable. For sheer output, I too would recommend the Titanic. But for actual SQ, regardless of the output, I'll stick to (and highly recommend) the HF or the HO.