Add sub to 802D4s?

Your friend is correct, don't make the mistake of going for too much bass, you just muddy the sound.

Unless you are willing to add 2 subwoofers and a digital 2 way crossover your friend is absolutely correct. The thump is actually not in the very low bass. 

Will you be missing anything important without subwoofers? If you like to listen at louder volumes and want the effect of feeling like you are at a live event then yes. If you listen at lower levels and do not care about the comparison to live concerts then no. At louder levels using subwoofers with a two way crossover removes the very low bass and very long excursions from the main speakers lowering distortion and increasing headroom by as much as 10 dB (twice as loud).  

Historically, classical lovers hated subwoofers and rockers loved them with jazz in between. 

I have B&W 802d3’s and have 2 JL Audio f 113’s.  I listen to rock, indie, and some alternative.  I love bass and the subs really give music an extra kick and dynamic.   I had them both in repair about a year ago and really missed the deeper bass.   As mentioned above, subs not really needed at lower volumes.   

A pair of subs, well implemented, will always improve sound significantly. The thing many people don’t realize is that good-quality subs not only improve bass response but also hugely improve imaging and soundstage. When you turn the subs off, the soundstage just collapses and you won’t wanna listen without them anymore. The difference is not small. Wilson uses their subs with their Alexandrias that stomp your B&Ws into the ground, so what does that tell you?  Your friend has absolutely no idea what he’s talking about and has clearly never heard good and well-implemented subs. And yes, you need at least two subs and preferably four that’s commonly called a swarm. Get the subs and you’ll never go back.

Ive personally owned that Mac amp,if you want those speakers to stand up and take notice dump it..Way better amps out there that can get you where you want with out adding subs,although I agree subs are a must in most systems..just my opinion....but start with a better amp.