Is this (88dB) enough?
No. Not really. Though again it could depend on the impedance of the speakers, the amp, and the music you listen to (string quartet chamber music might be possible, rock with any kind of bass and the answer may be no.
Efficient speakers are not (in my opinion) 90dB or better but more like 96-97dB or better. And even then, the quality of the bass can suffer if you don't have enough power (or damping factor) as bass simply can suck the life out of an amp - really strong good bass requires significant control of a large moving object. At 88dB and the S.E.X., string quartets might be possible but most movie soundtracks would be impossible to do well.
More efficient speakers require larger size. Larger size requires, at its most base case, more material to cover the outside of a larger internal volume, and often more consideration for bracing due to the greater flexibility created by larger surfaces. At the most basic level, more material requires more money. It can also require more work/time to do well, but if you DIY, it is a matter of materials (and plans). If you really want to go down the inexpensive DIY route for efficient speakers and can get materials cheaply, you might try a Sachiko (search "Sachiko" in this forum) or google "Sachiko speaker". They are quite efficient, by all accounts sound nice, and if you can get the material cheaply, shouldn't be that difficult to make.
System1: Jadis Eurythmies, Jadis JP-80, ZYX Artisan, Denon DP-100 (and others)
System2: Altec Model19s, EAR 859, Diatone LT-1, Sony SDC-777ES
currently replacing System2 electronics with BH Paramounts, FP3, Eros.
Listening Wear: BH baseball cap, obviously...