Well, that certainly is out of character for a Mullard.
I am curious, what kind of headphones are you using? There isn't anything listed in your profile.
Regardless of whom you purchase a vintage tube from, one unknown is how long it's been, since the tube was regularly used. Letting it run for a half hour is a good first step, but it might take a bit longer before it sounds better. Simply running it on a tube tester will tell you how good the tube is, but it isn't going to really exercise the tube like regular use will do.
Vacuum tubes are an interesting device. I spent 14-odd years of my life making very high-tech versions of vacuum tubes. They are finnicky, and their best performance is obtained by using them regularly, and often. Leaving them off for a prolonged period of time, can actually degrade their performance. You need to pick one of those tubes, and let it run for a while (meaning, days, at least) before you pass judgement on it. It may even take longer. There's no precise formula.
You bought a pair of 1967 Hammond rebranded Mullards. They are the "short" plate variety (meaning they are unlikely to be microphonic, but they are not as good as the 17mm long plate variety). In my experience, rebranded tubes (of any kind) are not quite as good as the original brand (while that is a highly subjective statement, it has been generally true for the several dozen varieties I've tried). You're also using late 60's tubes, which just aren't as good as tubes made 10 years prior (but not nearly as expensive either).
Two of these tubes for $35 bucks is a reasonable price for them. After you run them for a while (more than 30 minutes, perhaps more like several weeks), I would expect them to sound superior to the stock Shuguang crap that is shipped with the Bravo's. To me, the Shuguang tubes are thin, tinny, and shrill, compared to any vintage Mullard. But its difficult to compare a brand new tube that's been recently burned in, against a 45 year old tube that may have been sitting unused for decades.
If you want the absolute top quality Mullards, you need to look for the late 50's vintage D-getter tubes, and you will pay easily $100 each for them. Sometimes more. The sound quality of those particular tubes is simply amazing. I own a couple of them, and I paid quite a bit of money for them (more than my Bravo's cost). But I don't think we need to go down that road. You have a decent pair of 67 vintage tubes. Pick one of them, put it in your Bravo, plug some cans in (anything will do), turn the thing on, and let it run for a good long while (days at least), then give it a listen.