High-entropy alloys (HEAs) are a new class of materials constructed from multiple principal elements statistically arranged on simple crystallographic lattices. Due to the large amount of disorder present, they are excellent model systems for investigating the properties of materials intermediate between crystalline and amorphous states. Here we report the effects of systematic isoelectronic replacements, using Mo-Y, Mo-Sc, and Cr-Sc mixtures, for the valence electron count 4 and 5 elements in the body-centered cubic (BCC) Ta-Nb-Zr-Hf-Ti high-entropy alloy (HEA) superconductor. We find that the superconducting transition temperature T_c strongly depends on the elemental makeup of the alloy, and not exclusively its electron count. The replacement of niobium or tantalum by an isoelectronic mixture lowers the transition temperature by more than 60%, while the isoelectronic replacement of hafnium, zirconium, or titanium has a limited impact on T_c. We further explore the alloying of aluminium into the nearly optimal electron count [TaNb]_0.67(ZrHfTi)_0.33 HEA superconductor. The electron count dependence of the superconducting T_c for (HEA)Alx is found to be more crystallinelike than for the [TaNb]_1−x(ZrHfTi)_x HEA solid solution. For an aluminum content of x=0.4 the high-entropy stabilization of the simple BCC lattice breaks down. This material crystallizes in the tetragonal β-uranium structure type and superconductivity is not observed above 1.8 K.