I've never liked the K-701 with vocals, they sound unnatural. Most who own the K-501 and/or K-1000 find those much better than the K-701. Shame AKG pulled the plug on them. There are some expensive headphones that do vocals well, but the two affordable pairs I love are the Sennheiser HD-600 and the Beyerdynamic DT48.
Everyone seems to love the HD-600; you will find piles written about it and lots of positive reviews. You'd have a hard time going wrong with one. Its tonality is spot on and it is quite resolving. It's something of a "standard" here for good reason. The DT48 is terribly polarizing, though. People either love or hate it with almost no middle ground. The DT48 is one of the most resolving and transparent headphones available at any price. It plays bass notes but there's no "punch" to them. However, just going with female vocals you'll be stunned with its quality.
If you can find one used, you might want to consider the AKG K-501. For vocals, it is a superb headphone. When I directly compared the K-501 to the K-701 (I owned both), the K-501 really showed up the shortcomings of the K-701.
As for others, the DT880 seemes to have a little treble emphasis to me. I did like the pair I owned (2003 version), but they weren't the best I've heard. I didn't enjoy straight vocals on the RS-1 I had, either. That was more of a rock only phone.
In terms of an amp, I wouldn't buy an inexpensive tube amp. I think there are too many compromises in their power supply and build quality to provide good sound. You really have to lay a foundation with the power supply to get the most out of an amp. Solid state, however, is much less expensive to build a good lower supply for. Solid state runs under 24V most of the time, so far less expensive parts can be used. Tubes often run at 250V or more, so you have to use heavier components.
You can, however, build your own quality tube amp for $400 or less, but you won't find a commercial one. I think solid state is your best option.