We present a general framework for designing optimal transmit spectra for the HDSL2 service. Using the channel and interference transfer functions and SNR estimates, we set up and solve an optimization problem to maximize the capacity. Sizable gains in performance margins (or bit rates) result. Furthermore, by design, the spectra are spectrally compatible with other services. While 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 computationaly complexity. Our results can be used either for dynamically adapting the signaling spectra to account for changing noise or intereference conditions or for the design of new fixed transmit spectral masks using worst-case analysis.