Based on what you said about tastes in bass, the ATH-M50 might not be for you, then. It has a fairly neutral sound. Not the smoothest, as it is only $100-150. It does have a slight U-shaped curve to the sound signature, though. Still decently balanced, but the bass might not be for you. It's definitely worth a try, though.
The AD-700 is really, really bass light. I've heard some people say it's fairly balanced and punchy or even a bit rolled off but still punchy. Just...no. To me, it sounded completely limp and lifeless. Plus, the treble wasn't the smoothest (in a it hurt my ears kind of way). Objective measurements tend to side with what I heard.
Grado cans are, IMO, junk. They all measure about the same in terms of frequency response, distortion, etc. Hump around 100Hz with a slope down in the mids until a huge, gross, rough peak in the treble region. They also tend to have high harmonic distortion, especially in the bass. The harmonic distortion and large 100Hz hump actually makes them sound like they have more bass than they really do. The Grado SR60 hurt my ears due to the treble. Go look at measurements of all the Grado headphones from a variety of sources...it's mostly the same across all of the models.
The Philips Uptown probably sounds pretty good, but the measurements I've seen of it suggest it has a fairly large, but not overwhelming, boost in the bass. I tried one in Best Buy, and it did sound to have more of a U-shaped sound signature, but not in a displeasing way. However, testing a headphone at Best Buy means little.
I'm not sure what you read about the HD4x9 line. I had the HD429 for a while, and I found it to be fairly balanced across the spectrum, if not a bit laid back. Two measurements I've seen (from Innerfidelity and Golden Ears) suggest it is fairly neutral up until 1-2KHz, at which point the measurements start to differ. The pair I heard did not seem to have the large midrange dip measurements suggest, and the treble was not as rolled as you'd expect. I actually found them quite detailed and smooth without any noticeable harshness. Measurements for the three headphones in the HD4x9 line are very similar, though I have only personally listened to the HD429. I'm fairly certain the use the same drivers and housing...just different ear pads.
You might want to consider a used or refurbished Sennheiser HD558 if you are OK with open-back headphones (great for gaming due to large soundstage). I think you can get them for under $150 that way, if not closer to $100. They are fantastic budget headphones, and they actually sound and measure fairly neutral. Very natural, open sound. Bass extends down low and is punchy if the music calls for it, but it is not emphasized (neutral or even slightly rolled off, but absolutely the bass is NOT emphasized). Mids and treble are smooth and well balanced across the board. Actually, that's where the HD598 excels...mids, treble, and soundstage. They actually measure very similarly to the HD600, and even more so after an easy mod (remove a piece of foam from inside the cup...easy). The main difference is that the HD558 and HD598 have higher (but not bad) harmonic distortion than the HD600.
Be careful about Head-Fi. There's a lot of good information on there, but the large percentage of users on there wouldn't recognize good, neutral sound if it smacked them upside the head. It's OK, though, if you are open about your tastes and pick headphones based on that. I'm talking about people who just have bad ears. There are too many zealots on there that are too easily sold on products, and even the worst products will get praise from a large number of people. Some shun objective measurements and analysis of headphones entirely, and some shun the subjective side. It's a very, very weird community of people more or less in a large circle jerk for headphones. However, there are also many, many intelligent, reasonable people on there with great ears and solid backgrounds in the audio fields that actually have fantastic advice and insight. The hard part is figuring out who to listen to and who to tune out...
Feel free to PM me if you have any questions. You might benefit from utilizing objective measurements along with subjective impressions to narrow down on what headphone will suit your needs and tastes best.