Increased competition in the centre of defence is high on Sunderland manager Sam Allardyce's summer wishlist.
And the Black Cats are understood to be tracking in-demand QPR centre-back Steven Caulker and Borussia Dortmund defender Neven Subotic.
Caulker, who has two-years left on his reported £35,000-a-week wages, is said to be available for a cut-price £4.5million fee.
But Sunderland - also linked with a move for West Ham striker Diafra Sakho - face stiff competition with Newcastle United, West Brom and Crystal Palace all monitoring Caulker.
With Wes Brown set to leave when his contract expires, it leaves Allardyce with just John O'Shea, Younes Kaboul and Lamine Kone in the centre.
Stoke are also said to be keeping tabs on Serbian defender Subotic, with a season-long loan or a cut-price fee two options attractive to Sunderland.
The defender, keen on a move to the Premier League, is adamant he has not made any firm decisions about his future though, with Dortmund talks planned.
Subotic - who made just 11 appearances last season - said: "We had a phenomenal season and I understand the coach cannot keep everyone happy.
"But I did not become a footballer to get paid for warming the bench.
"I really appreciate the club, the team and the fans. But we will definitely sit down to discuss my future.
"Then I have to look at what the club want me to do and if that fits with my ideas.
"If that is not the case, you have to have an open mind. But we are not there yet."
Sunderland are also understood to be tracking two highly-rated youngsters in the Football League; Walsall defender Rico Henry and Leeds United wideman Charlie Taylor.
Birmingham-born Henry started his career at the Bescot Stadium and has worked his way through the club’s youth system before breaking into the first team, making 44 appearances last season, scoring three goals.
An England Under-19 international, the highly-sought after 18-year-old has two years left on his existing contract at Walsall.
Taylor, 22, is a product of the Leeds youth academy and has broken into the Elland Road first team, playing 43 games last season.
The former England Under-19 international can play as a left winger or left-back, an area Allardyce is keen to strengthen.
Meanwhile, Torino are understood to have joined Lazio, Bologna and Fiorentina in the race to sign Emanuele Giaccherini.
The winger is set to represent Italy at Euro 2016 and is also wanted by Watford.
And Sunderland outcast Charis Mavrias looks set to be heading for the exit door on a permanent basis.
Mavrias spent the second half of the 2015-16 campaign on loan at German second tier outfit Fortuna Dusseldorf and a permanent switch is on the cards after they avoided relegation.
The 21-year-old Greece international doesn't have a future on Wearside after just seven appearances for the club.