which is a better speaker

I agree they both kind of stink. Have very little experience with the Pioneers, but all older JBLs I have heard are pretty bad sounding.

I am not familiar with the equipment but based on comments above that it is designed for rowdy reproduction of rock I would recommend Zu Audio.  You don't mention what your budget is but they have extremely reasonable offerings for every budget.  You can also find them used online at times.  I do not own a pair but have read many, many positive reviews.  They offer 60 day in home trial which addresses some of the recommendations of others.

@mike317 

To be fair, as posed your question is difficult to answer meaningfully.  What kind of music do you like to listen to, at what volumes and in what size of a room? Are you listening in one position or moving around?  Can you describe what kind of sound you like?  Without more information, the best anyone could do is look at the specifications for the equipment in question to see if there are any glaring mismatches, especially with regards to impedance and power.  There are lots of charts online showing frequency response, impedance (I didn't see any in a quick search but I'm sure they exist), off axis response, etc.  

Both speakers are 8 ohms, a good load for this receiver.  I don't have their respective impedance curves to spot any weirdness, but I would think it unlikely for mass market bookshelf speakers to have crazy impedance dips that could stress a power supply.  

The Pioneers are 92.5dB efficient with 50W power handling; the JBL are 89dB with 250W power handling.  So they are fairly close in output.  The Marantz will be able to get more volume from the HPM-100s because it doesn't have the juice to push the JBL to their limits.  

I have heard the Marantz 2265b and many of its brethren.  I grew up with a 2252B but many years later found I preferred the 2270 for it's more euphonic sound.

I have heard neither speaker, so can't comment on their respective sound signatures and how those might or might not jibe with the Marantz.  But they are both very well known.  The Pioneer was designed by Bart Locanthi, I believe, and was very well received.  The JBL is from a line with a tremendous pedigree, culminating in the 250ti, which I think is an "audiophile" speaker by today's standards.

I will however, echo something a previous poster noted, which is that if your receiver is all original, it will definitely not be sounding up its potential.  Electronic components age and, if you love this receiver, investing in an overhaul will keep it running nicely for decades to come.

It's been said that "you can't blame a compass for pointing north".  And, when you make an inquiry to a group of audiophiles you can expect "audiophile" responses from "audiophiles".  If the question were posed on a forum for oral surgeons: "What toothpaste should  I buy -- Colgate or Crest?" you'd expect a strong critique of mainstream, available at Target/Walmart oral hygiene products as well as some professional recommendations of sensible alternatives.  There ARE actual audio experts on this forum.  And, the price of admission for that advice is reasonable.  Free.

My part time "gig" after retirement involves servicing vintage audio equipment.  The primary goal is to prevent good gear from going into the dumpster.  We are indifferent to brand, price point, overall condition, etc. Our primary prerequisite is that the piece must have a strong emotional attachment to its owner.  It cost us about $3 for each $1 we bring in.  Not a great return on investment. Being there at times like when a retired eye surgeon breaks out into tears after we fire up his father's stereo that's been in storage for 50+ years, is our reward. 

Vintage "stuff" brings us back to an era of authenticity, but it's more than that.  For some of us, the journey from childhood to adulthood was enhanced by music and the hardware that produced the music.  Some of the best of the best memories of our lifetimes involved those knobs, switches, levers, meters, dials, and fret wood grilles. While it might a stretch to call them a literal fountain of youth, our old gear certainly does return us to "feel good" moments that are unique to that equipment.  Referencing, again, the service component mentioned above, I'm sure that in 90% of the cases listed above, if the owner(s) were offered 10x the value their equipment in exchange much better sounding modern gear, the answer would be a resounding "NO!"

Seeking a "rational decision" involving elements with an emotional attachment is an irrational approach in that it is impossible to place a value on the those elements. 

That being said, the OP's comments about the group, in general, was a bit heavy handed.  I am genuinely impressed with this group's commitment to serving others.  These are the kinda guys I'd like to have as neighbors when my riding mower won't start.  They'd be there to pull the plug, rebuild the carb, etc.  Then, offer ME a cold one after THEY've done all the work.

As, yes, the HPM100s are pretty special.

Just a note to my A'Gon friends:  Mainstream vintage gear can take on real audiophile characteristic with a little attention to parts quality and application of "newer thinking".  We do this routinely and are often surprised (stunned!) with the results.

There are tons of speakers as well as reviews of same. Just decide what you want, stand mount or floor standing, then read lots of reviews about them and narrow the field to a few. Then go listen if possible. If not possible, trust the reviews if many seem to agree, and take a chance on a pair. Most things can be returned in 30 to 60 days.....