This paper presents a general framework for designing optimal transmit spectra for symmetric bit-rate communication services dominated by crosstalk, in particular Digital Subscriber Line (DSL) services such as the proposed HDSL2. Using the channel, noise, and interference transfer functions, we set up and solve an optimization problem to maximize the joint capacity of neighboring lines. Joint signaling techniques and optimal power distribution yield significant gains in bit rates (or performance margins) over current schemes. Furthermore, by design, the spectra are spectrally compatible with existing neighboring services. The framework is quite general --- it does not depend on the exact choice of modulation scheme, for example. It is also extremely simple and of low computational complexity.