Sounds like you've made a decision. I listen to similar genres of music in JPop (YUI, Stereopony) and orchestral (Final Fantasy, Fairy Tail soundtracks). The headphones I've gone through are in order of acquisition: ATH-M50, HD598, HD700, Beyerdynamic T90, Hifiman HE-500, Koss ESP950, HD800, and AKG K812. The amps I've gone through in order of acquisition: JDS Labs O2, Woo Audio WA7, Bottlehead Crack, and Questyle CMA800R. My DACs: JDS Labs ODAC, WA7 DAC, Firestone Audio Spitfire HD, Audioengine D3, Schiit Modi, and Emotiva DC-1.
While I have not heard the T1, I owned the T90 which is supposedly the baby T1, and many who have owned both say it's 90% the T1. I have not owned the HDVD800 either, but I found myself to stay away from amp/DAC combos after owning both the O2/ODAC and WA7. The reasoning for this is that you'll inevitably want to upgrade one of the components, and that part of it goes to waste in your combo. The WA7, for example, has a great amp section, but the DAC is mediocre. Really a $200 DAC at best for a $1000 combo device. I believe the HDVD800 has a similar problem where the DAC is also quite mediocre, and I'd recommend you get the HDVA600 instead if you really want the Sennheiser amp. The HDVD800 and HDVA600 has the exact same amp section, with the HDVA600 being a standalone amp. It's a good $500 cheaper, and really the DAC in the HDVD800 is a $200 DAC at best.
Once again, I want to reiterate that I have not heard the T1 or the HDVD800/HDVA600.
I got into this hobby about a year ago. Actually purchased the M50 last March. I soon upgraded to the HD598 and was enthralled by the open headphone sound. I immediately wanted to upgrade to something better. I went for the HD700 and the WA7. I bought each at the full price of $1000 each. But soon as I went on to other headphones, I realized how foolish a mistake that was. The HD700 was not worth $1000, and neither was the WA7 imo. The HD700 soon dropped to $650, and I ended up selling it for $500, half of what I paid for. I soon learned from then on to only buy when there is a deal. I don't know how much you paid for the T1, but it was listed on two different websites the past two months for only $750 new. You can read some more of my experiences in detail here:
Even that I wrote a couple months ago. I recently did the math on how much I've spend since getting into this hobby about a year ago. The raw amount I spent, not counting money made selling some stuff later on, was about $12,000. Yes that's more than the cost of Stax SR-009 + BHSE. Again, the actual amount I spend is somewhat less at around $8000-9000 if you consider the money I made back from selling gear I moved on from. Currently, I am very satisfied with my headphone and amp combo. I'm using the HD800 and Questyle CMA800R. The MSRP of each is $1500, but because I'm more seasoned now, and look out for deals, I got the HD800 at $890 and the Questyle at $1000, both new. This is cheaper than what I paid for HD700 and WA7. And to me, it's easily twice as good in sound quality. How much I would have loved to know what I know now and pocket back that $6000 and just gotten the HD800 and Questyle in the first place.
Now for me personally, I find my current HD800 and CMA800R to be the best I've heard and great for both JPop and Orchestral. The pairing takes away any brightness concerns the HD800 may have. Mids are the smoothest I've heard, especially with female vocals.
I just wanted to share my personal experience, because I see you are in a similar situation I was. I wish I has someone to tell me ll this back then. Personally, I think getting the HDVD800 out of convenience for the DAC/amp combo is a rookie mistake, as is buying the T1 at any price higher than $800.
Note: Don't get the Questyle for the T1. It was built for the HD800, and really only sounds good with the HD800. If you really like the HDVD800, get the HDVA600 instead. Even then at $1500, you have plenty of other options for the amp. If you have any soldering experience and wouldn't mind trying tube amps, look into Bottlehead. They sell DIY kits that cut away any of the labor costs. The Bottlehead Crack at $300 plays on par with many amps in the $1000 range, and should pair well with the T1. The Bottlehead Mainline is their top of the line amp at $1200, and I'd imagine it to compete with some of the $2000-$3000 amps such as the offerings by Eddie Current. Even then, you can get these kits at a sale price. I've seen the Mainline sell for $1000 during a sale they had, and I bought my Crack with the Speedball included for free ($120 value).
This isn't super relevant to the OP, but have you ever tried the HD650's? If you like the open Sennheiser sound, I think the darling of the Sennheiser lineup (HD650) could be worth getting to replace the HD700's. ...which i know sounds like a downgrade, but I don't often see reviews of the HD700's that say they're worth the price. The HD650's on the other hand, are highly recommended by everyone, despite all the early talk of being dark or veiled. People actually LIKE the dark/veiled sound. It's not actually that veiled at all. The highs are just slightly rolled, but they're actually quite good, and paired with a hybrid, or even better, a full tube amp, like the crack, they're amazing. If you want a completely balanced set of headphones that have a massive soundstage, the HD800's won't be beat, but for a set of headphones with warmth and character, I don't know if you can beat the HD650's.
I keep seeing people in the HD650 thread say that they own HD800's, T1's, LCD-2's, etc, and they still find themselves going back to their HD650's. I've never heard that kind of praise for the HD700's, so I figured I'd mention if you haven't already thought about it, that it might be worth selling the 700's and picking up some 650's. Just planting a seed! No pressure lol. You already own the amp with the best synergy with the HD650's so it just makes sense!