If you are only using the notes below an E occasionally then keeping a four tuned BEAD that you can switch to for songs that need it may be a better option than you think. Or if you use notes down to D a lot and lower notes not so much as myself and several others who have commented on this recently have found, the compromise tunings like drop D, DADG, noted above or D standard, DGCF, could be a good option for you. Converting one of your basses to an alternate tuning and selling one will probably be financially better than selling two and buying a fiver. And if the alternate tuning would work for all your playing then selling two and buying none will put your DB fund over the top even quicker. I don't think you would need to mod the nut to try the two D based tunings either.