What next?

The next step would be a streamer and DAC. Sounds like you are talking a very low budget. Have a look at a BlueSound Node X. This should be a huge jump in performance. 
 

The more you spend (assuming careful choosing) the better sound you will get up to the limit of your amplification / speakers. If I was in your situation I would go as far as a Schiit Yggdrasil (no I am not joking) DAC, and a used Aurrender N100 (or better yet N150) Streamer. That is my guess about where you would be enjoying the best sound  quality from your system. Which should be far far better than what you get now. Using an iPhone as a source is pretty much the lowest level.

Agree your next move should be a streamer and DAC.  Not sure what your budget is, but you could get a highly-regarded iFi Zen Stream and Musician Draco DAC for about $1000 and be at an entirely higher performance level.  I started streaming with my iPhone, but when I added the Zen Stream my streaming sound quality went to another level entirely — not even close.  Best of luck.

Thanks for the replies, will be researching them.

BTW, what is your budget and would you want new or used?

You are so far getting fantastic advice…. ditch Apple music as fast as possible…

Node X is kind of spendy, if you don't need the added features, pick up a used Node 2i

I have restored a Marantz PM94 and have recently purchased a pair of Von Schweikert VR4.5 speakers, am using Apple Music through my iPhone. What would be recommended to improve the sounds? Also have a restored Sansui 9090DB with stacked Advent speakers. Just read on the forum about the value of preamps and wondering if I should be looking at a tube preamp to add to my systems? Am retired so not looking to spend big bucks. Thanks in advance for suggestions.

Your marantz is a integrated amp and already came with a preamp.

To make the sound better, you can get an affordable full function streamer+DAC like the Wiim pro plus for 200 bucks.

https://www.crutchfield.com/p_399WIIMPPL/WiiM-Pro-Plus.html

If you are operating on a restricted wallet, look at the "Cheap Audio Man" YT channel. The guy generally talks about affordable/ relatively higher value gear.

Budget $1-2,000 am ok with used if reviews show it worthwhile. 

Schiit Freya + (I use the original version) and maybe a Bifrost 2/64...both sound first class and are somewhat "el-cheapo a mundo."

You’re where I was 6 years ago, great speakers, crappy front end.  Buy a used Innuos Zen Mk3 and a decent DAC like a used Schiit Yggy.  They’ll make you happy!

As mentions by others herein, $2000 would make for a great improvement over use of a iphone with a purchase of a streamer/DAC. You will get a greater return on your investment in the amp and speakers.

If you can expand your price range a bit, an open box Bluesound Node N130 and a new or used Denifrips Aries 2 will make a huge improvement. You might also think about getting rid of Apple Music and going with Qobuz, Amazon Music HD or Tidal.

All the best.

Others have suggested, pick up a new or used Bluesound Node 130; used is about $350....not a Node 2i, there is only about a $30 difference at used prices; it is several years newer in technology.

Add a used Denafrips Pontus II DAC for about $1,300, or a Denafrips Ares II DAC for about $700.....but I'd strongly suggest stretching for the Pontus II if funds are available, it's a really really strong DAC for the money.

Here are links to DAC and streamer I mentioned above…
https://www.amazon.com/Musician-Draco-Entry-Level-Decoder-DSD1024/dp/B096S6HBQY/ref=sr_1_2?crid=3QZMWR4WC4TMP&keywords=musician+draco&qid=1707851084&sprefix=Musician+draco%2Caps%2C360&sr=8-2

And here’s a good review of the DAC…

https://soundnews.net/sources/dacs/musician-audio-draco-r-2r-ladder-dac-review/

https://www.amazon.com/iFi-Zen-Stream-Transport-Ethernet/dp/B095HN5QK8/ref=sr_1_2?crid=21S1FUKSXFFZV&keywords=IFi+zen+stream&qid=1707851212&sprefix=ifi+zen+stream%2Caps%2C338&sr=8-2&ufe=app_do%3Aamzn1.fos.c3015c4a-46bb-44b9-81a4-dc28e6d374b3

So for about $1000 you’re at a completely higher level of streaming performance.  BTW, if you do get the Zen Stream you should strongly consider this upgraded power supply as it greatly enhances performance…

https://www.amazon.com/iPower-Noise-Power-Supply-Adapter/dp/B0847T82LK/ref=sr_1_3?crid=25CUSDI5OT5NZ&keywords=ifi%2Bipower%2Bx&qid=1707851905&sprefix=IFi%2Bipowe%2Caps%2C329&sr=8-3&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0&th=1