[Mr Rick]

  Hi Mr Rick - my issue with the 598 is that it's somewhat distant (?not as engaging as more forward headphones) - does that have to do with the drop of the Senns in the 1k - 5k segment? Both my Grado and Shure don't have that drop ... and are therefore more forward? Or is that unrelated? (And I guess it's more complex anyway....)
cheers

Lets look at a comparison of your 598 and the HD600.
 

You will note that both headphones follow a similar curve. Some might call this the "Sennheiser Sound"  
 
Starting about 30 hz both headphones are very similar out to about 1000hz. At that point both headphones start to fall off, with the 598s down about 2db to the 600s.
 
The real differences occur at 10,000hz, where the 598 falls about 10db. 
 
The dip from 1 to 5 Khz will be noticeable compared to headphones without the dip.  As you can see, the 600s will be better and perhaps will be more forward.
 
The real value of a frequency response graph is the ability to compare differences to other headphones. I use them all the time to find headphones that will sound "different" than the ones I already own. I'm not a believer in one headphone sounding "better" then another. Better is so subjective, and will vary from person to person and is very dependent on the music material used and equipment driving the headphones. All come into play to form what you hear.
 
If you would like to perform your own comparisons, here is the site that I use for that purpose.
 
 http://www.headphone.com/pages/build-a-graph

 

[nojdrof]

I've had my hd700s for about 2 weeks and love them. Very detailed and seem to reproduce the recordings accurately. They're a little light in bass volume but make up for it in accuracy and the bass is tight and realistic. I'm using an Integra 40.1 preamp tuner to power these but I'm sure I will see an improvement with a nice tube headphone amp (any recommendations welcome).

 

[Click]

I've had my hd700s for about 2 weeks and love them. Very detailed and seem to reproduce the recordings accurately. They're a little light in bass volume but make up for it in accuracy and the bass is tight and realistic. I'm using an Integra 40.1 preamp tuner to power these but I'm sure I will see an improvement with a nice tube headphone amp (any recommendations welcome).

Welcome to head-fi. Glad you're enjoying HD700.

Seems like a lot of new HD700 owners / posters have been popping up recently due to the newer, lower average selling price. 
 
If I had to be picky, the only downside with HD700 is that you don't get the rumbling bass that certain closed-back headphones and good IEMs have. Then again, it was never designed to be a basshead can.
 
However, like you said, HD700 reproduces high quality bass. Tight, controlled, precise, and hits hard. Quality over quantity.

 

[nojdrof]

Glad to be here, thank you. For $499.00 I couldn't find anything I liked better. It could be my imagination but I think the bass is getting better as I use them. I've also found that it really makes a huge difference with high quality recordings. I'd like to get into tube amps is there anything you'd recommend in the $500 range? Thanks again for the welcome!

 

[gibosi]

Off the top of my head,
 
If you are willing to build a kit, The Bottlehead Crack
 
http://bottlehead.com/product/crack-otl-headphone-amplifier-kit/
 
For a bit more, an Elise (Feliks Audio)
 
http://www.feliksaudio.pl/en
 
Or a custom built Glenn OTL (which I have)
 
http://www.head-fi.org/t/600110/2359glenn-studio#post_8211572

 

[Click]

Glad to be here, thank you. For $499.00 I couldn't find anything I liked better. It could be my imagination but I think the bass is getting better as I use them. I've also found that it really makes a huge difference with high quality recordings. I'd like to get into tube amps is there anything you'd recommend in the $500 range? Thanks again for the welcome!

Yeah, HD700 scales up with better DAC and amps. Quality of recordings also makes a big difference in overall sound quality. 
 
As for amps, there are a lot of suggestions in here if you can sift through months and years worth of posts. Under $500, DavidA always recommends Project Ember for HD700. He's paired HD700 with tons of amps before and says that's the best pairing with HD700. I've been happy with Vali, but if I ever upgrade the amp, it would be that one.

 

[gibosi]

Not to take anything away from the Project Ember, but that amp is a hybrid, that is, a tube front end driving an op amp output. The amps I suggested above have tubes in the driver and the output positions, slightly different animals, so to speak. If hybrids are under consideration, than I would suggest that you add the Schiit Lyr to your list.

 

[DavidA]

@gibosi, @Click, only reason I think the Ember is better than the BH Crack for the HD-700 @150 Ohms is that it has better synergy with the Ember, the Lyr2 will also work quite well but for me the Ember gives me the best sound.  The BH Crack will work well but it seems match better with the higher Z headphones that I have like HD-600/650/800, T1, DT-990, everyone has there own taste in what they like so some might say that an all tube design sounds better than a hybrid and I would accept that.
 
FWIW, I've also heard the HD-700 driven by EC Balancing Act, WA6, WA22, Rag, McIntosh MHA-100, and a few other high end amps while in Japan visiting my friend a few months ago and they do scale with these, but at the prices of these amps I would rather get more headphones, but that is just me.
 
PS: having tubes for driver and output gets expensive really fast as I have found even with just the BH Crack, one reason I haven't gotten the Elise or WA2

 

[Krutsch]

Glad to be here, thank you. For $499.00 I couldn't find anything I liked better. It could be my imagination but I think the bass is getting better as I use them. I've also found that it really makes a huge difference with high quality recordings. I'd like to get into tube amps is there anything you'd recommend in the $500 range? Thanks again for the welcome!

Seriously consider a Woo Audio WA3. It's at the high end of $500.00, but in my experience, it's a better experience than the BottleHead (from my own listening) and, as others have mentioned, you really want a true OTL, which is the WA3, as opposed to the Ember.
 
Just my $0.02...

 

[gibosi]

And I would agree that for 150 ohm cans, the BH is borderline....
 
I have a hybrid Little Dot 1+ and an all-tube Glenn OTL. I am quite sure my LD is not as good as the Project Ember, but then, it costs only half as much. That said, for a bit more than $100, the LD delivers a lot of value for the price and it makes a great introduction to the wonderful and enchanting world of tubes. And I have to believe that for a bit more money, a Project Ember or a Lyr would provide an even better experience.
 
And yes, if one makes the decision to go with an all-tube amp, especially one with vacuum tube rectification, the road to becoming a tube-aholic is a very steep and slippery downhill slope. And unfortunately, Tube-aholics Anonymous chapters are simply non-existent.... And I speak from experience, with over 1000 tubes and more on their way! lol.
 
In my experience an OTL with two 6AS7G-type power tubes can be a terrific amp for the HD700.
 

 

[Krutsch]

  And I would agree that for 150 ohm cans, the BH is borderline....
 
I have a hybrid Little Dot 1+ and an all-tube Glenn OTL. I am quite sure my LD is not as good as the Project Ember, but then, it costs only half as much. That said, for a bit more than $100, the LD delivers a lot of value for the price and it makes a great introduction to the wonderful and enchanting world of tubes. And I have to believe that for a bit more money, a Project Ember or a Lyr would provide an even better experience.
 
And yes, if one makes the decision to go with an all-tube amp, especially one with vacuum tube rectification, the road to becoming a tube-aholic is a very steep and slippery downhill slope. And unfortunately, Tube-aholics Anonymous chapters are simply non-existent.... And I speak from experience, with over 1000 tubes and more on their way! lol.
 
In my experience an OTL with two 6AS7G-type power tubes can a terrific amp for the HD700.
 

Nice looking amp... so, is the Glenn OTL in the $500 price range? 

 

[gibosi]

  Nice looking amp... so, is the Glenn OTL in the $500 price range? 

 
The last time I checked (last spring) the base price was $650, so a bit more, but in my opinion, a bargain.

 

[Krutsch]

   
The last time I checked (last spring) the base price was $650, so a bit more, but in my opinion, a bargain.

Including tubes...? Wow, that's not bad.

 

[gibosi]

  Including tubes...? Wow, that's not bad.

 
I believe so...  He asked me what I needed, and I told him I didn't have any rectifiers, so he sent one. As I already had a good assortment of drivers and power tubes, I really didn't need any of those from him.

 

[Mechans1]

The out put stage of an amp or the power amps of a larger system must be tube to get the real magic of tube amplification.  I own an Ember and yes it is fun to roll the tubes but, try an amp with tube power not SS and you will see what I mean (I hope anyway)