A joint analysis of spectroscopic data obtained at liquid-helium temperatures by three line-narrowing techniques, photon echo (PE), persistent hole burning (HB), and single molecule spectroscopy (SMS), is presented. Two polymer systems, polyisobutylene (PIB) and polymethylmethacrylate (PMMA), doped with tetra-tert-butylterrylene (TBT) were studied via PE and HE techniques and the results are compared with literature data [R. Kettner pr al., J. Phys, Chem. 98, 6671 (1994); B. Kozankiewicz er al., J. Chem. Phys. 101, 9377 (1994)] obtained by SMS. Both systems behave quite differently. In TBT/PIB a rather strong influence of a dispersion of the dephasing time T-2 was found which plays only a minor role in TBT/PMMA. We have also measured the temperature dependence of T-2 for both systems in a broad temperature range (0.4-22 K). Using these data we separated the two different contributions to the optical dephasing - due to an interaction with two-level systems and due to coupling with local low-frequency modes. The data are compared with calculations using a numerical and a semianalytical model in the presence of a large dispersion of the single molecule parameters. Furthermore, we discuss the differences of the linewidths as measured by different experimental methods.